feat(flutter): send timezone on login + app-start + weekly

Flutter client posts FlutterTimezone.getLocalTimezone() to
PUT /api/me/timezone on every setSession (login / register success)
and on every AuthController.build (app cold-start with valid
session), when the locally-stored tz_last_sent_at is >7 days old.
Cadence tracked in flutter_secure_storage so it survives app
restarts.

Failures swallowed: the server's UTC default + last-known value
keep the scheduler functioning until the next attempt.

Completes the client side of #392 Half B (per-user timezone
scheduling).

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

flutter_client/pubspec.lock

@@ -411,6 +411,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_timezone:
+    dependency: "direct main"
+    description:
+      name: flutter_timezone
+      sha256: "13b2109ad75651faced4831bf262e32559e44aa549426eab8a597610d385d934"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.1"
   flutter_web_plugins:
     dependency: transitive
     description: flutter