Add app lock, multi-account support, and redesigned account settings

App lock (local_auth) gates the app behind the OS's biometric/device
credential prompt via a new lock screen, layered on top of the existing
Nextcloud session rather than replacing it.

Multi-account support lets users save several Nextcloud logins and switch
between them (AccountStore, per-account browsing prefs, avatar swipe-to-
cycle); logout() now keeps an account's saved credentials/prefs intact so
it can be resumed from a "Continue as..." list, while removeAccount()
stays the only destructive path.

The account settings screen is redesigned around a single profile card
(avatar/name/email, server host, storage quota with groups) with
Refresh Cache/Logout/Remove actions, replacing the old separate "Server
Connection Info" section.

Also fixes activity-tab timestamps: the OCS Activity API reports time as
an ISO 8601 `datetime` string, not the numeric `timestamp` field the code
incorrectly assumed, which made every activity render as Jan 1 1970.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -1,5 +1,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
     <uses-permission android:name="android.permission.INTERNET"/>
+    <!-- For local_auth's biometric app-lock (Settings > Security). -->
+    <uses-permission android:name="android.permission.USE_BIOMETRIC"/>
     <application
         android:label="Noo"
         android:name="${applicationName}"

android/app/src/main/kotlin/dev/ayushya/noo/MainActivity.kt

@@ -1,5 +1,8 @@
 package dev.ayushya.noo
 
-import io.flutter.embedding.android.FlutterActivity
+import io.flutter.embedding.android.FlutterFragmentActivity
 
-class MainActivity : FlutterActivity()
+// FlutterFragmentActivity (not the default FlutterActivity) is required by
+// local_auth's Android implementation, which hosts its biometric/device
+// credential prompt via a Fragment.
+class MainActivity : FlutterFragmentActivity()