migrate timer to ForegroundService for background survival

Move CountDownTimer logic from a Dialog into a ForegroundService so
the timer survives when the user tabs away or the activity is
destroyed. The activity becomes a thin UI projection bound to the
service via TimerStateListener callbacks.

- Create CountdownService with notification, state persistence,
  and ToneGenerator/Vibrator ownership
- Persist timer state to SharedPreferences for process death recovery
- Add notification with progress bar and Stop/Restart actions
- Embed countdown UI in activity_main.xml alongside picker UI
- Delete CountdownDialog and dialog_countdown.xml
- Update AndroidManifest with foreground service permissions

.devcontainer/devcontainer.json

@@ -9,7 +9,8 @@
                 "felipecaslazana.vscode-android-logcat",
                 "ms-azuretools.vscode-docker",
                 "fwcd.kotlin",
-                "vscjava.vscode-java-pack"
+                "vscjava.vscode-java-pack",
+                "Anthropic.claude-code"
             ]
         }
     },