## 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
