refactor(desktop): make Tauri thin shell running web sidecar (#273)

## What / Why
This PR finishes the desktop refactor: the Tauri app is now a thin shell that launches the web server as a sidecar and loads the UI from `http://127.0.0.1:<port>`. All real backend logic lives in `packages/web/server/index.js`; desktop Rust keeps only native integrations (menu/dialog/notifications/updater/deep-link + window chrome).
This unblocks:
- consistent behavior across web/desktop/vscode (single backend)
- simpler desktop maintenance (no duplicated Rust backend)
- host switching between Local + remote instances in desktop
- reliable cold-start behavior on slow machines (VSCode + desktop)
## Key changes
- Desktop sidecar runtime
  - build pipeline to bundle web dist + `openchamber-server` sidecar (`packages/desktop/scripts/build-sidecar.mjs`)
  - robust local port selection (prefer saved/default, fallback to random; persisted in `~/.config/openchamber/settings.json`)
  - improved PATH handling so the sidecar can locate `opencode` CLI (incl `~/.opencode/bin`, overrides, common bins)
  - disable native right-click context menu in production builds (dev keeps it)
- Desktop instance switcher (Tauri-only)
  - header button + modal to add/edit/delete remote hosts, set default, probe status/ping, switch back to Local escape hatch
  - auth gate includes host switcher so you can recover when a remote host is broken/auth-required
  - host list stored desktop-locally (not tied to the currently selected remote server)
- Notifications
  - decision logic moved server-side; desktop notifications emitted via sidecar stdout and shown natively by Tauri
  - prevent double-notifications on desktop Local origin (UI ignores SSE notification when native path is active)
  - restore macOS notification sound
- Updates
  - Tauri updater used only when viewing Local instance in desktop shell (avoid “remote web update” triggering desktop restart)
- Settings persistence & UX polish
  - persist model favorites/recents via `/api/config/settings` (works for web + desktop; not origin-dependent)
  - persist per-project sidebar collapse state in `projects[].sidebarCollapsed` via `/api/config/settings` (with debounce on toggles)
  - macOS header sizing/traffic-lights offsets fixed (marketing macOS major injected from desktop; MultiRun header aligned)
  - VSCode cold-start: keep retrying provider/agent loads after connection to avoid empty UI on slow machines
  - misc lint/type fixes + bun.lock sync
- Desktop bootstrap / resiliency
  - show onboarding screen when OpenCode CLI is missing (desktop Local origin), with retry hook to restart OpenCode after install
## Testing notes
- Desktop (macOS): switch Local <-> remote, set default host, verify auth gate recovery, native notifications (with sound), updater gated to Local
- Web: favorites/recents + per-project collapsed state persist across reload/restart
- VSCode: slow startup no longer results in missing providers/agents/models

packages/desktop/src-tauri/Cargo.toml

@@ -4,10 +4,6 @@ version = "1.6.3"
 edition = "2021"
 publish = false
 
-[lib]
-name = "openchamber_desktop"
-path = "src/lib.rs"
-
 [[bin]]
 name = "openchamber-desktop"
 path = "src/main.rs"
@@ -18,46 +14,18 @@ devtools = ["tauri/devtools"]
 
 [dependencies]
 anyhow = "1.0.86"
-axum = { version = "0.8.4", features = ["macros"] }
-chrono = { version = "0.4", features = ["serde"] }
-dirs = "5.0"
-fastrand = "2.0"
-futures-util = "0.3"
 log = "0.4.28"
-nix = { version = "0.28", features = ["signal"] }
-objc = "0.2.7"
-objc2 = "0.6.3"
-objc2-foundation = { version = "0.3.2", features = ["NSProcessInfo", "NSString", "NSObjCRuntime"] }
-once_cell = "1.19"
-parking_lot = "0.12.3"
-portable-pty = "0.9.0"
-portpicker = "0.1.1"
-regex = "1.10.4"
-reqwest = { version = "0.12.4", default-features = false, features = ["json", "stream", "rustls-tls", "gzip", "brotli", "deflate"] }
+reqwest = { version = "0.12.4", default-features = false, features = ["rustls-tls"] }
 serde = { version = "1.0.210", features = ["derive"] }
 serde_json = "1.0.143"
-serde_yaml = "0.9"
-json5 = "0.4"
 tauri = { version = "2.9.4", features = ["macos-private-api"] }
 tauri-plugin-dialog = "2.4.2"
-tauri-plugin-fs = "2.4.4"
 tauri-plugin-log = "2.7.1"
 tauri-plugin-shell = "2.3.3"
-tokio = { version = "1.38", features = ["macros", "rt-multi-thread", "process", "signal", "sync", "time", "fs"] }
-tower-http = { version = "0.5.2", features = ["cors"] }
-url = "2.5"
-uuid = { version = "1.18.1", features = ["v4"] }
-tokio-util = { version = "0.7", features = ["io"] }
 tauri-plugin-notification = "2.3.3"
 tauri-plugin-updater = "2"
-tauri-plugin-process = "2"
-base64 = "0.22.1"
-urlencoding = "2.1"
-zip = "2.1"
+tokio = { version = "1.38", features = ["rt-multi-thread", "time"] }
+url = "2.5"
 
 [build-dependencies]
 tauri-build = { version = "2.5.3", features = [] }
-
-[target.'cfg(target_os = "macos")'.dependencies]
-objc2-app-kit = { version = "0.3.2", features = ["NSView", "NSResponder"] }
-objc2-quartz-core = { version = "0.3.2", features = ["CALayer"] }