Portrait / landscape display-rotation setting (x86 + Raspberry Pi) #2856

Anthias does not currently expose a display-rotation setting. The Qt webview renders at the OS-level orientation, so users on portrait monitors (particularly on x86) end up with content sideways, and mixed-orientation playlists rotate portrait images to landscape rather than keeping each asset in its original orientation.

This issue consolidates and supersedes:

• Rotation not right, not possible to fix on x86 [BUG] #2693 — Rotation not right, not possible to fix on x86
• [BUG] If image rotation inconsistent, portrait images are rotated to landscape #2326 — If image rotation inconsistent, portrait images are rotated to landscape
• Can’t get Anthias working in Portrait 9:16 mode #1997 — Can't get Anthias working in Portrait 9:16 mode
• would like to see a setting for using the screen in Portrait or Landscape mode  #1545 — Setting for Portrait/Landscape mode

Scope

• A persisted rotation setting (0 / 90 / 180 / 270 degrees) exposed on the Settings page.
• Applied at the display-output level on both supported targets:
  • Raspberry Pi 3/4 (Qt5 webview)
  • Pi 5 / x86 (Qt6 webview)
• Honoured by both image and video assets.
• Mixed-orientation playlists keep each asset in its original orientation rather than auto-rotating to fit the dominant orientation.

Out of scope (track separately)

• Per-asset rotation override.
• Auto-detect EXIF orientation on upload — better suited to the existing asset-processing pipeline (feat(processing): normalise HEIC/HEIF/TIFF images and exotic-codec videos at upload time #2832).

Implementation notes

• No screen_rotate / rotation flag is read anywhere in webview/ or settings today; this needs a new persisted setting plus plumbing through the Qt webview launch.
• On x86 / Bookworm under labwc, rotation is typically a wlr-randr / compositor concern, so we may need to apply rotation at the compositor level rather than inside Qt itself. The Qt5 path on Raspberry Pi (DRM/KMS) likely needs a different approach.