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-tauri/src/models/settings.rs

@@ -0,0 +1,29 @@
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Settings {
+    pub confirm_before_delete: bool,
+    pub default_folder_id: Option<String>,
+    pub theme: String,
+    pub font_size: String,
+    pub default_ports: HashMap<String, Option<i64>>,
+    pub tag_order: Option<String>,
+    pub table_refresh_rate: i64,
+    pub table_page_size: i64,
+    pub shortcuts: HashMap<String, String>,
+    pub accent_color: String,
+    // Editor (Plan A)
+    pub editor_font_size: i64,
+    pub editor_font_family: String,
+    pub editor_word_wrap: String,
+    pub editor_minimap: bool,
+    pub editor_tab_size: i64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SettingsExport {
+    pub schema_version: u32,
+    pub settings: Settings,
+}