Add slideshow mode to the live view

[matteius]

Summary

Adds an AnyCam-style slideshow to the live view. A new toolbar toggle auto-advances the grid on a timer:

• 1×1 layout: cycles through cameras (drives the stream selector), wrapping at the end.
• Larger grids: cycles through pages, wrapping at the end.
• Respects the active tag / collection / favorites filters and custom camera order.
• Interval picker (5 / 10 / 15 / 30 / 60s, default 10s) appears while on.
• Persisted per browser and mirrored to the URL as ?slideshow=<seconds> (?slideshow=0 turns it off), so kiosks / wall displays can bookmark e.g. /?cols=1&rows=1&slideshow=15.
• Pauses during tile reorder, per-tile native fullscreen, the building-plan surface, and while the tab is hidden. Keeps running in page fullscreen. Manual paging/selection restarts the countdown.
• Toggle is hidden when there's nothing to cycle.

Wired into both LiveView (HLS/MSE) and WebRTCView. New strings in en and fil; other locales fall back to English.

Known limitation

Each slide change tears down and reconnects streams, same as clicking Next, so there's a ~1s load between slides (longer on HLS). Pre-warming the next slide's streams is a possible follow-up.

Test plan

• npx jest: 47 suites / 368 tests pass
  • tests/liveSlideshow.spec.js: preference parsing, URL override, next-slide/wrap logic
  • tests/liveSlideshowHook.spec.js: hook timer behaviour via a deps-aware preact/hooks shim (page and camera cycling, interval, countdown reset on manual navigation, no reset on same-camera refetch, pause/resume, hidden tab, nothing-to-cycle, storage + ?slideshow= sync)
  • tests/liveSlideshowControl.spec.js: control rendered via esbuild; labels, aria-pressed, interval options, callbacks
  • Mutation-checked: dropping the pause/visibility guard, the countdown-reset deps, the refetch keying, the wrap modulo, or the URL cleanup each fails at least one test
• npx vite build succeeds
• Manual check in a browser: 1×1 camera cycling, 2×2 page cycling, ?slideshow= bookmark, pause on reorder/fullscreen tile

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