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/build.gradle.kts

@@ -18,7 +18,10 @@ android {
         applicationId = "dev.ayushya.noo"
         // You can update the following values to match your application needs.
         // For more information, see: https://flutter.dev/to/review-gradle-config.
-        minSdk = flutter.minSdkVersion
+        // local_auth (app-lock/biometric) requires API 24+; Flutter's own
+        // default may be lower, so floor it here rather than relying on
+        // whatever flutter.minSdkVersion currently resolves to.
+        minSdk = maxOf(24, flutter.minSdkVersion)
         targetSdk = flutter.targetSdkVersion
         versionCode = flutter.versionCode
         versionName = flutter.versionName