perf: cut integration-ui test time from 250s to 28s

- Fix HOME override that caused FVM to re-download 220MB Flutter SDK on
  every run; use XDG_DATA_HOME instead to isolate app data without
  touching HOME
- Switch DB path from getApplicationDocumentsDirectory() to
  getApplicationSupportDirectory() so XDG_DATA_HOME isolation works and
  stale accounts don't leak between test runs
- Replace fixed pump(5s/3s) waits with pumpUntil() polling at 200ms so
  tests stop waiting as soon as the UI is ready (23s of dead wait → 8s)
- Add timing instrumentation (ts() in shell, _log()/Stopwatch in Dart)
- Fix CI integration-ui job: was mixing subosito flutter with fvm flutter;
  now uses fvm consistently with actions/cache for ~/.fvm, ~/.pub-cache,
  and build/linux

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -47,17 +47,25 @@ jobs:
 
       - uses: DeterminateSystems/magic-nix-cache-action@v8
 
-      - uses: subosito/flutter-action@v2
+      - name: Cache FVM Flutter SDK
+        uses: actions/cache@v4
         with:
-          flutter-version: "3.41.6"
-          channel: stable
-          cache: true
+          path: ~/.fvm
+          key: fvm-${{ hashFiles('.fvm/fvm_config.json') }}
+
+      - name: Cache pub packages
+        uses: actions/cache@v4
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ hashFiles('pubspec.lock') }}
+          restore-keys: pub-
 
       - name: Run integration tests
         run: |
           nix develop --command bash -c "
-            flutter pub get &&
-            flutter pub run build_runner build --delete-conflicting-outputs &&
+            fvm install --skip-pub-get &&
+            fvm flutter pub get &&
+            fvm flutter pub run build_runner build --delete-conflicting-outputs &&
             stalwart-dev/test.sh
           "
 
@@ -80,17 +88,34 @@ jobs:
             libgtk-3-dev pkg-config cmake ninja-build clang \
             libsecret-1-dev
 
-      - uses: subosito/flutter-action@v2
+      - name: Cache FVM Flutter SDK
+        uses: actions/cache@v4
         with:
-          flutter-version: "3.41.6"
-          channel: stable
-          cache: true
+          path: ~/.fvm
+          key: fvm-${{ hashFiles('.fvm/fvm_config.json') }}
+
+      - name: Cache pub packages
+        uses: actions/cache@v4
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ hashFiles('pubspec.lock') }}
+          restore-keys: pub-
+
+      - name: Cache Linux debug build
+        uses: actions/cache@v4
+        with:
+          path: |
+            build/linux
+            .dart_tool/flutter_build
+          key: linux-debug-${{ hashFiles('pubspec.lock', 'lib/**/*.dart', 'integration_test/**/*.dart') }}
+          restore-keys: linux-debug-
 
       - name: Run UI integration tests
         run: |
           nix develop --command bash -c "
-            flutter pub get &&
-            flutter pub run build_runner build --delete-conflicting-outputs &&
+            fvm install --skip-pub-get &&
+            fvm flutter pub get &&
+            fvm flutter pub run build_runner build --delete-conflicting-outputs &&
             stalwart-dev/integration_ui_test.sh
           "