feat(timer): handle rotation in-app without relaunching activity

[Xitee1]

Summary

• Lock MainActivity to portrait and detect device pose via OrientationEventListener, then rotate text/icons inside Compose so the UI stays readable when the phone is tilted — the dial no longer overflows on system rotation.
• Landscape layout: title moves from TopBar centre to the right edge (inline with the settings icon), and "Endet um" slides down via translationY so it sits midway between the dial and the buttons. The required shift is computed from BoxWithConstraints, so the dial stays put on any screen size.
• Reverse portrait (180°): same physical layout as portrait, just every element rotated 180° — the text reuses exactly the same space it had upright.
• All rotating elements share one animated angle (shortest-path aware, 350ms tween) so the effect reads as a single synchronised rotation.

Design doc: docs/plans/2026-04-18-in-app-rotation-design.md.

Test plan

• Tilt the phone 90° in either direction on the timer screen: dial contents rotate, title appears next to the settings icon on the right edge, "Endet um" sits centred between dial and buttons, all button icons rotate.
• Tilt to 180° (upside down): title stays in the TopBar centre and "Endet um" stays in its portrait position, just rotated 180°.
• Rotate between the four orientations several times: rotation always takes the shortest visual path without snap-back.
• Navigate to settings / about and back: rotation state recovers correctly; those screens remain portrait-only.
• Verify on a smaller device that the dial position doesn't shift when "Endet um" slides.

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