fix: Amber login on mobile with two-phase clipboard flow

Mobile browsers block navigator.clipboard.readText() unless called
inside a user gesture (tap/click). The old flow relied on the
visibilitychange event to auto-read the clipboard when the user
returned from Amber, which silently failed.

New flow:
1. User taps "Sign in with Amber" → opens Amber via Android intent
2. User approves in Amber → pubkey copied to clipboard
3. User returns to browser → sees "Complete Sign-in" button
4. User taps "Complete Sign-in" → clipboard read succeeds (user gesture)
5. Pubkey decoded, account registered, backend session created

Also handles npub/nprofile decoding and provides clear error messages
for empty clipboard, missing permissions, and non-Android devices.

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

docker-compose.yml

@@ -20,7 +20,7 @@ services:
       context: .
       dockerfile: Dockerfile
       args:
-        CACHEBUST: "16"
+        CACHEBUST: "17"
         VITE_USE_MOCK_DATA: "false"
         VITE_CONTENT_ORIGIN: ${FRONTEND_URL}
         VITE_INDEEHUB_API_URL: /api