fix(frontend): preserve unusual video aspect ratios

[hmans]

Summary

• preserve posted videos at their intrinsic display aspect ratio without cropping
• remove the old 16:9 presentation heuristic and its cover crop overrides
• override Vidstack's internal 16:9 default so normal ratios use the measured
  attachment frame with contain
• clamp only the player canvas to 9:16–16:9 for extreme media, letterboxing the
  complete video so Vidstack's controls retain usable space
• retain the existing maximum width and height so every player remains bounded
  in the timeline
• document the behavior and tradeoff in FDR-008

Verification

• mise lint-frontend
• mise x -- pnpm exec vitest --run --project=client src/lib/components/chat/VideoPlayer.svelte.spec.ts (8 tests)
• Svelte autofixer for VideoPlayer.svelte
• Prettier check for changed files
• git diff --check

Closes #1520.