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>

pubspec.lock

@@ -278,6 +278,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.0.0"
+  flutter_plugin_android_lifecycle:
+    dependency: transitive
+    description:
+      name: flutter_plugin_android_lifecycle
+      sha256: "3854fe5e3bff0b113c658f260b90c95dea17c92db0f2addeac2e343dd9969785"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.0.35"
   flutter_secure_storage:
     dependency: "direct main"
     description:
@@ -456,6 +464,46 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.1.0"
+  local_auth:
+    dependency: "direct main"
+    description:
+      name: local_auth
+      sha256: "434d854cf478f17f12ab29a76a02b3067f86a63a6d6c4eb8fbfdcfe4879c1b7b"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.3.0"
+  local_auth_android:
+    dependency: transitive
+    description:
+      name: local_auth_android
+      sha256: a0bdfcc0607050a26ef5b31d6b4b254581c3d3ce3c1816ab4d4f4a9173e84467
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.56"
+  local_auth_darwin:
+    dependency: transitive
+    description:
+      name: local_auth_darwin
+      sha256: "699873970067a40ef2f2c09b4c72eb1cfef64224ef041b3df9fdc5c4c1f91f49"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.1"
+  local_auth_platform_interface:
+    dependency: transitive
+    description:
+      name: local_auth_platform_interface
+      sha256: f98b8e388588583d3f781f6806e4f4c9f9e189d898d27f0c249b93a1973dd122
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.0"
+  local_auth_windows:
+    dependency: transitive
+    description:
+      name: local_auth_windows
+      sha256: bc4e66a29b0fdf751aafbec923b5bed7ad6ed3614875d8151afe2578520b2ab5
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.11"
   logging:
     dependency: transitive
     description: