feat(settings): add screen rotation mode setting

[Xitee1]

Summary

Adds a rotation-mode setting so users can control screen orientation, which was previously hard-wired (Timer screen locked to portrait with in-Compose counter-rotation; other screens followed the device pose — ignoring the system-wide auto-rotate lock entirely).

A new Auto-rotate row in Settings → Appearance opens a dialog (same pattern as the existing screen-lock dialog) with three modes:

• System setting (new default) — rotate only while the OS auto-rotate flag is on, observed via Settings.System.ACCELEROMETER_ROTATION (no new permission; the app stays network/permission-free).
• Always — rotate with the device even when system rotation is locked (the prior behavior; useful lying in bed with rotation locked).
• Portrait — never rotate.

The mode is stored in DataStore as an AutoRotateMode enum (mirroring the existing ThemeId pattern) and threaded through rememberDeviceOrientation(mode), which gates the OrientationEventListener — and, in System mode, a ContentObserver on the system flag. Everything downstream (the window lock in AppOrientationController, the nav-fade guard, and the timer's in-Compose counter-rotation) follows from the resolved orientation, so no per-call-site branching was needed.

Implementation notes

• New files: AutoRotateMode enum, AutoRotateModeDialog (mirrors ScreenLockMethodDialog, with the selected option accent-tinted per the theme-picker's selection language), and AppOrientationViewModel (lets the ViewModel-less SleepTimerNavHost observe the setting).
• The ContentObserver only exists in the composition while in System mode, so it self-disposes when the mode changes; it's process-scoped, so quick-settings flips made while backgrounded still land before the user returns.
• Strings added to both values/ and values-de/.

Verification

• ./gradlew assembleDebug lint — green.
• Grep-confirmed no source references to old symbols remain.
• Not yet run on a device. Worth exercising there: all three modes × system-rotate on/off, mid-session quick-settings flips (foreground + backgrounded-then-return), and persistence across restart.

🤖 Generated with Claude Code

[Xitee1]

Closes #38