feat(09-05): ThemeSessionBridge + ThemeToggle write-through to /api/me/theme

- Create ThemeSessionBridge.tsx: useEffect compares session.user.theme to
  next-themes value; calls setTheme(serverTheme) on mismatch; validates
  against 3-string allowlist ('light'|'dark'|'system'); renders null
- Mount <ThemeSessionBridge /> as first child of <AuthProvider> in app/layout.tsx
- Modify ThemeToggle: writeTheme() calls setTheme() then fire-and-forget
  PUT /api/me/theme; silent catch for network errors (best-effort desktop UX)

app/layout.tsx

@@ -7,6 +7,7 @@ import { CommandPalette } from "@/components/navigation/command-palette";
 import { TaglineFooter } from "@/components/branding/tagline-footer";
 import { Toaster } from "sonner";
 import { AuthProvider } from "@/components/auth/auth-provider";
+import { ThemeSessionBridge } from "@/components/mobile/profile/ThemeSessionBridge";
 
 // IBM Plex Sans replaces the brand-mandated Helvetica/Arial.  The 2013
 // standards guide called for Helvetica Bold for headers and Helvetica
@@ -69,6 +70,7 @@ export default function RootLayout({
           disableTransitionOnChange
         >
           <AuthProvider>
+            <ThemeSessionBridge />
             <div className="min-h-screen bg-background flex flex-col">
               <AppNavigation />
               <main className="flex-1">{children}</main>