fix(icons): stop macOS drawing the app icon on a legacy plate (#40)

On macOS 26+ the Dock and any launcher that resolves app icons through
NSWorkspace.icon(forFile:) (Spotlight, Vicinae, Raycast, ...) drew a grey
rounded-square "legacy plate" behind Gridline's icon, i.e. a white border
around the app icon at every render size below ~72px.

Reproduced outside any launcher: NSWorkspace.icon(forFile:) on
Gridline.app returns the plated icon at 16-64px and the clean icon at
>=72px. The trigger lives in icon.icns, not in the app -- moving that file
into Zed.app reproduces the border, moving Zed's icns into Gridline.app
removes it.

Two things in the file cause it:

  - ic12 (64x64)                     -> plates the 40-64px range
  - is32/s8mk (16x16 RGB+mask)
    il32/l8mk (32x32 RGB+mask)       -> plates the 16-32px range

macOS treats padded small representations as legacy, and this icns carries
them in both places.

Fix, in two parts.

1. Strip those chunks. scripts/fix-icns.mjs does it; `tauri icon` re-emits
   them on every run, so it has to be a committed, repeatable step.
   scripts/verify-app-icon.mjs is the regression check: it builds a
   throwaway .app around the committed icon.icns, asks AppKit to render it
   at 16..512px and fails if any size comes back plated. It fails (6/11
   plated) on the previous icns and passes on this one.

2. Regenerate the raster set from icon.svg via
   scripts/regenerate-icons.sh, so the icons directory matches its source
   of truth again. The SVG has been full-bleed since the initial commit
   (md5 unchanged at d872e42/3dab09f/6854d88/HEAD) while the rasters were
   replaced with a hand-tuned padded version at v0.2.0 and had drifted
   from it ever since. The script generates into a temp dir because
   `tauri icon` only takes one output directory and would otherwise drop
   generated android/ and ios/ trees into the desktop-only icons dir.

No visual change on macOS: IconServices normalizes app-icon artwork when
rendering, so padded and full-bleed sets come out pixel-identical there
(same bbox and art fraction at 48/128/512px).

Windows and Linux do consume the PNG/ICO directly, so the icon there does
change from padded to full-bleed -- consistent with the SVG, but a
visible difference worth eyeballing before release.

Verified: verify-app-icon passes at 16, 24, 32, 40, 48, 64, 72, 96, 128,
256, 512px; iconutil and `file` still decode the new .icns and .ico
(6 sizes); frontend tests pass (131 files / 1134 tests).

scripts/verify-app-icon.mjs

@@ -0,0 +1,147 @@
+#!/usr/bin/env bun
+/**
+ * Verify that the repo's macOS app icon is not being rendered on a "legacy
+ * plate" — the grey/white rounded-square border macOS 26+ draws behind app
+ * icons at small sizes (Dock, Spotlight, Vicinae, Raycast, …).
+ *
+ * Usage:
+ *   bun scripts/verify-app-icon.mjs
+ *
+ * Builds a throwaway .app around desktop/src-tauri/icons/icon.icns, asks AppKit
+ * (`NSWorkspace.icon(forFile:)` — the same API launchers use) to render it at a
+ * range of sizes, and samples the pixels to detect the plate. Exits non-zero if
+ * any size comes out plated.
+ *
+ * This is the regression check for `fix-icns.mjs`: `tauri icon` re-adds the
+ * offending representations every time it runs, so run this (or
+ * `scripts/regenerate-icons.sh`, which calls the fix) after touching icons.
+ *
+ * macOS-only. Requires Swift (Xcode or the Command Line Tools).
+ */
+import { spawnSync } from "node:child_process";
+import { existsSync, mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const REPO_ROOT = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const ICNS = join(REPO_ROOT, "desktop/src-tauri/icons/icon.icns");
+
+if (process.platform !== "darwin") {
+  console.log("verify-app-icon: not macOS, nothing to check");
+  process.exit(0);
+}
+
+if (!existsSync(ICNS)) {
+  console.error(`verify-app-icon: missing ${ICNS}`);
+  process.exit(1);
+}
+
+const work = mkdtempSync(join(tmpdir(), "gridline-icon-check-"));
+process.on("exit", () => rmSync(work, { recursive: true, force: true }));
+
+// The synthetic bundle just needs to be a directory with an Info.plist pointing
+// at the icon. A unique path + bundle identifier each run keeps IconServices'
+// cache from serving a previous run's verdict.
+const appDir = join(work, "IconCheck.app");
+mkdirSync(join(appDir, "Contents", "Resources"), { recursive: true });
+writeFileSync(join(appDir, "Contents", "Resources", "icon.icns"), readFileSync(ICNS));
+writeFileSync(
+  join(appDir, "Contents", "Info.plist"),
+  `<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+  <key>CFBundleIdentifier</key><string>com.gridline.iconcheck.${process.pid}${Date.now()}</string>
+  <key>CFBundleName</key><string>IconCheck</string>
+  <key>CFBundleIconFile</key><string>icon.icns</string>
+  <key>CFBundlePackageType</key><string>APPL</string>
+</dict>
+</plist>
+`,
+);
+
+// Samples the pixel just inside the icon square at 12% height. A legacy plate
+// there is a neutral light grey; the artwork itself is dark at that point.
+const SWIFT = `
+import AppKit
+
+let appPath = CommandLine.arguments[1]
+let sizes = CommandLine.arguments[2].split(separator: ",").compactMap { Int($0) }
+let image = NSWorkspace.shared.icon(forFile: appPath)
+
+for px in sizes {
+  let rep = NSBitmapImageRep(bitmapDataPlanes: nil, pixelsWide: px, pixelsHigh: px,
+    bitsPerSample: 8, samplesPerPixel: 4, hasAlpha: true, isPlanar: false,
+    colorSpaceName: .calibratedRGB, bytesPerRow: px * 4, bitsPerPixel: 32)!
+  let ctx = NSGraphicsContext(bitmapImageRep: rep)!
+  NSGraphicsContext.saveGraphicsState()
+  NSGraphicsContext.current = ctx
+  ctx.imageInterpolation = .high
+  NSColor.clear.setFill()
+  NSRect(x: 0, y: 0, width: px, height: px).fill(using: .copy)
+  let natural = image.size
+  let scale = min(CGFloat(px) / natural.width, CGFloat(px) / natural.height)
+  let w = natural.width * scale
+  let h = natural.height * scale
+  image.draw(in: NSRect(x: (CGFloat(px) - w) / 2, y: (CGFloat(px) - h) / 2, width: w, height: h),
+             from: .zero, operation: .sourceOver, fraction: 1.0, respectFlipped: true, hints: nil)
+  NSGraphicsContext.restoreGraphicsState()
+
+  let probe = rep.colorAt(x: px / 2, y: max(1, Int(Double(px) * 0.12)))
+  let r = probe?.redComponent ?? 0
+  let g = probe?.greenComponent ?? 0
+  let b = probe?.blueComponent ?? 0
+  let a = probe?.alphaComponent ?? 0
+  let v = Int((r * 255).rounded())
+  let plated = a > 0.78 && abs(r - g) < 0.04 && abs(g - b) < 0.05 && v > 170 && v < 245
+  print("\\(px) \\(plated ? "PLATED" : "OK") v=\\(v)")
+}
+`;
+
+const swiftFile = join(work, "probe.swift");
+writeFileSync(swiftFile, SWIFT);
+const sizes = [16, 24, 32, 40, 48, 64, 72, 96, 128, 256, 512];
+
+function runSwift(env) {
+  return spawnSync("swift", [swiftFile, appDir, sizes.join(",")], {
+    encoding: "utf8",
+    env: { ...process.env, ...env },
+  });
+}
+
+// `swift` from a full Xcode install refuses to run until its licence is accepted;
+// the Command Line Tools toolchain is a drop-in fallback for a plain AppKit program.
+let result = runSwift({});
+if (result.error || result.status !== 0) {
+  const clt = "/Library/Developer/CommandLineTools";
+  if (existsSync(join(clt, "usr/bin/swift"))) {
+    result = runSwift({ DEVELOPER_DIR: clt });
+  }
+}
+
+if (result.error || !result.stdout) {
+  console.error("verify-app-icon: could not run Swift/AppKit");
+  console.error(result.error?.message ?? result.stderr?.trim() ?? "");
+  process.exit(1);
+}
+
+const lines = result.stdout.trim().split("\n").filter((l) => l.includes("PLATED") || l.includes("OK"));
+const plated = lines.filter((l) => l.split(" ")[1] === "PLATED");
+
+console.log("macOS icon render check (NSWorkspace.icon(forFile:))");
+for (const line of lines) {
+  const [size, verdict] = line.split(" ");
+  console.log(`  ${size.padStart(4)}px  ${verdict === "PLATED" ? "PLATED — grey legacy plate" : "ok"}`);
+}
+
+if (plated.length > 0) {
+  console.error(
+    `\nFAIL: ${plated.length}/${lines.length} sizes render on a legacy plate.\n` +
+      "The icns still carries the legacy representations (ic12 / is32 / s8mk / il32 / l8mk).\n" +
+      "Run: bun scripts/fix-icns.mjs && bun scripts/regenerate-icons.sh",
+  );
+  process.exit(1);
+}
+
+console.log(`\nPASS: clean at every tested size (${sizes.join(", ")}px).`);