feat(desktop): macOS vibrancy for the left sidebar with a toggle

Add native macOS vibrancy behind the left sidebar (the only translucent
surface; header/chat/right sidebar stay opaque), plus a setting to turn it off.

- Window created with vibrancy applied after first show (avoids the cold-launch
  no-composite quirk); minimize/restore suppress the frost during the genie
  animation. Renderer frosts the sidebar via --sidebar-vibrancy-overlay once
  data-oc-vibrancy[-ready] are set; project-actions pill matches when open.
- data-oc-vibrancy-ready defaults are set in cssGenerator (DOM guaranteed),
  not the preload (document-start race left the sidebar un-frosted on launch).
- prefers-reduced-transparency falls back to solid surfaces.
- Appearance settings (macOS desktop only): a checkbox to enable/disable
  vibrancy, persisted to settings.json and applied via a Save & restart button
  (vibrancy is a window-creation option, so it needs a relaunch).

packages/electron/preload.mjs

@@ -17,6 +17,10 @@ const clientToken = readArgValue('--openchamber-client-token');
 const homeDirectory = readArgValue('--openchamber-home');
 const macosMajorRaw = readArgValue('--openchamber-macos-major');
 const macosMajor = Number.parseInt(macosMajorRaw, 10);
+const macVibrancySupported = process.platform === 'darwin';
+// Effective state for this window (main process resolves the saved preference
+// and passes it in). Defaults on when supported unless explicitly '0'.
+const hasMacVibrancy = macVibrancySupported && readArgValue('--openchamber-mac-vibrancy') !== '0';
 
 // Preload re-executes on every cross-origin navigation (we run with
 // sandbox:false, per-document). Two separate concerns to balance:
@@ -72,6 +76,8 @@ if (Number.isFinite(macosMajor) && macosMajor > 0) {
 
 contextBridge.exposeInMainWorld('__OPENCHAMBER_ELECTRON__', {
   runtime: 'electron',
+  macVibrancy: hasMacVibrancy,
+  macVibrancySupported,
 });
 
 contextBridge.exposeInMainWorld('__OPENCHAMBER_PLATFORM__', process.platform);
@@ -120,6 +126,18 @@ const dispatchNativeEvent = (event, detail) => {
   }
 };
 
+// Toggles the frost on/off in response to the main process around the
+// minimize/restore cycle. The default ("ready") state is set reliably in the
+// renderer (cssGenerator) — not here — because this preload runs at
+// document-start when documentElement may not exist yet.
+const setVibrancyReady = (ready) => {
+  if (!hasMacVibrancy) return;
+  try {
+    document.documentElement.toggleAttribute('data-oc-vibrancy-ready', ready === true);
+  } catch {
+  }
+};
+
 // Main-process events are read-only notifications (update progress,
 // window focus, etc.) — safe to deliver to any page rendered in this
 // webContents. The events themselves don't grant capability.
@@ -133,6 +151,10 @@ ipcRenderer.on('openchamber:emit', (_evt, payload) => {
     return;
   }
 
+  if (event === 'openchamber:vibrancy-ready') {
+    setVibrancyReady(payload.detail?.ready === true);
+  }
+
   dispatchNativeEvent(event, payload.detail);
 });