Fix Live Activity lock screen rendering and polish multi-account auth.

Flatten widget layouts and use system colors so banner and expanded regions render on vibrant lock screens; migrate auth sessions per account to prevent double sign-in; scope app lock PIN to accounts; default clock description to "Clock In"; add architecture docs and deferred form validation on auth screens.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/TopChromeBar.tsx

@@ -10,7 +10,7 @@ import {
   TOP_CHROME_ROW_HEIGHT,
 } from "@/lib/top-chrome-insets";
 
-/** Blurred status-bar chrome with logo + clocked-in indicator. */
+/** Blurred status-bar chrome with logo + account switcher. */
 export function TopChromeBar() {
   const insets = useSafeAreaInsets();
   const { isDark } = useAppTheme();