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/TopChrome.tsx

@@ -1,19 +1,19 @@
 import { StyleSheet, View } from "react-native";
 
-import { ClockedInIndicator } from "@/components/ClockedInIndicator";
+import { AccountSwitcher } from "@/components/AccountSwitcher";
 import { Logo } from "@/components/Logo";
 import { spacing } from "@/constants/theme";
 import { useAppTheme } from "@/contexts/ThemeContext";
 import { TOP_CHROME_ROW_HEIGHT } from "@/lib/top-chrome-insets";
 
-/** Wordmark left, clocked-in indicator right — sits on TopChromeBar blur. */
+/** Wordmark left, account switcher right — sits on TopChromeBar blur. */
 export function TopChrome() {
   const { isDark } = useAppTheme();
 
   return (
     <View style={styles.row}>
       <Logo size="xs" onDark={isDark} />
-      <ClockedInIndicator />
+      <AccountSwitcher />
     </View>
   );
 }