chore(mobile): phase 1 quality-review fixups

Three small follow-ups from the post-Phase-1 quality review:

  * authContext.tsx — drop the dead `userId` extraction + the `void
    userId` suppressor that masked an unused-locals warning. The session
    is already implicitly threaded through the supabase client, so no
    consumer of ensureDevice needed the value.
  * authContext.tsx — switch the device-name string concat to a
    template literal for consistency with the rest of the codebase.
  * ErrorBoundary.tsx — replace the four inline hex literals with their
    `theme/colors.ts` constants. The boundary was authored in Phase 0
    before the theme module existed; this brings it in line with every
    Phase 1 screen.
  * apps/mobile/README.md — drop the stale Phase-0 paragraph about the
    `lib/sharedSmoke.ts` canary (deleted in Phase 1) and add a short
    pointer to the env-var setup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/mobile/components/ErrorBoundary.tsx

@@ -1,6 +1,8 @@
 import React from 'react';
 import { Pressable, StyleSheet, Text, View } from 'react-native';
 
+import { colors } from '../theme/colors';
+
 // Catch render errors anywhere below this boundary and show a readable
 // fallback. Without it, a thrown error during render produces a white
 // screen on TestFlight / production builds with no way for the user to
@@ -53,16 +55,16 @@ const styles = StyleSheet.create({
     flex: 1,
     alignItems: 'center',
     justifyContent: 'center',
-    backgroundColor: '#0b0b0f',
+    backgroundColor: colors.bg,
     padding: 24,
   },
-  title: { color: '#fff', fontSize: 22, fontWeight: '600', marginBottom: 8 },
-  message: { color: '#9ca3af', textAlign: 'center', marginBottom: 24 },
+  title: { color: colors.text, fontSize: 22, fontWeight: '600', marginBottom: 8 },
+  message: { color: colors.textMuted, textAlign: 'center', marginBottom: 24 },
   button: {
-    backgroundColor: '#5865f2',
+    backgroundColor: colors.accent,
     paddingHorizontal: 20,
     paddingVertical: 12,
     borderRadius: 10,
   },
-  buttonText: { color: '#fff', fontWeight: '600' },
+  buttonText: { color: colors.text, fontWeight: '600' },
 });