refactor: convert to bun-workspaces monorepo (desktop/ + www/)

- Move the Tauri app (React frontend + Rust backend) into desktop/ via
  git mv — configs unchanged (relative paths: tauri.conf.json, vite.config.ts)
- Root package.json becomes a private bun workspace container with
  orchestration scripts; desktop package renamed gridline-desktop
- Scaffold www/ with Astro 7 + Tailwind v4 (hand-wired vite plugin,
  static output, ready for Dokploy)
- Update release.yml for the new layout: projectPath: desktop,
  desktop/src-tauri resource paths, rust-cache workspace path
- Update README + AGENTS.md structure trees and dev commands
- Verified: vitest (1074), cargo test (354), desktop build, tauri dev
  (window launches), tauri build (dmg + app bundles), Astro build

desktop/src/components/connections/ProviderTabsGrid.tsx

@@ -0,0 +1,40 @@
+import { PROVIDER_TABS, SETUP_GUIDES, type ProviderId } from "../../lib/providers";
+import { DbIcon, ProviderIcon } from "../../lib/dbIcons";
+
+interface ProviderTabsGridProps {
+  selectedId?: ProviderId | null;
+  onSelect: (id: ProviderId) => void;
+}
+
+export function ProviderTabsGrid({ selectedId, onSelect }: ProviderTabsGridProps) {
+  return (
+    <div data-testid="provider-grid" className="grid grid-cols-2 gap-3">
+      {PROVIDER_TABS.map((p) => {
+        const isSelected = selectedId === p.id;
+        return (
+          <button
+            key={p.id}
+            type="button"
+            onClick={() => onSelect(p.id)}
+            aria-label={p.label}
+            className={`flex flex-col items-center gap-2 p-4 rounded-lg border bg-surface transition-colors cursor-pointer ${
+              isSelected
+                ? "border-accent text-text"
+                : "border-border text-text-muted hover:border-accent/50 hover:text-text"
+            }`}
+          >
+            <span className="w-8 h-8 flex items-center justify-center">
+              {p.isManagedPreset ? <ProviderIcon id={p.id as "supabase" | "neon"} size={24} /> : <DbIcon type={p.dbType} size={24} />}
+            </span>
+            <span className="text-sm font-medium">{p.label}</span>
+            {p.isManagedPreset && (
+              <span className="text-[10px] text-text-muted text-center leading-tight">
+                {SETUP_GUIDES[p.id as keyof typeof SETUP_GUIDES].blurb}
+              </span>
+            )}
+          </button>
+        );
+      })}
+    </div>
+  );
+}