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/ui/src/index.css

@@ -56,6 +56,10 @@ textarea[data-chat-input="true"]:focus-visible {
 
 [data-scroll-shadow="true"][data-orientation="vertical"] {
   mask-mode: alpha;
+  -webkit-mask-repeat: no-repeat;
+  mask-repeat: no-repeat;
+  -webkit-mask-size: 100% 100%;
+  mask-size: 100% 100%;
 }
 
 [data-scroll-shadow="true"][data-orientation="vertical"][data-top-bottom-scroll="true"] {
@@ -66,6 +70,13 @@ textarea[data-chat-input="true"]:focus-visible {
     #000 calc(100% - var(--scroll-shadow-size)),
     transparent 100%
   );
+  -webkit-mask-image: linear-gradient(
+    to bottom,
+    transparent 0%,
+    #000 var(--scroll-shadow-size),
+    #000 calc(100% - var(--scroll-shadow-size)),
+    transparent 100%
+  );
 }
 
 [data-scroll-shadow="true"][data-orientation="vertical"][data-top-scroll="true"] {
@@ -75,6 +86,12 @@ textarea[data-chat-input="true"]:focus-visible {
     #000 var(--scroll-shadow-size),
     #000 100%
   );
+  -webkit-mask-image: linear-gradient(
+    to bottom,
+    transparent 0%,
+    #000 var(--scroll-shadow-size),
+    #000 100%
+  );
 }
 
 [data-scroll-shadow="true"][data-orientation="vertical"][data-bottom-scroll="true"] {
@@ -84,6 +101,12 @@ textarea[data-chat-input="true"]:focus-visible {
     #000 calc(100% - var(--scroll-shadow-size)),
     transparent 100%
   );
+  -webkit-mask-image: linear-gradient(
+    to bottom,
+    #000 0%,
+    #000 calc(100% - var(--scroll-shadow-size)),
+    transparent 100%
+  );
 }