feat: U/I keys and "Highlights only" mode to review top-scored sections

J/K still steps through every queued section in time order; U/I steps
through only the highlights, defined as the top-N sections by score
(new "Top" input in the clip bar, default 50, matching the day chips).
A "Highlights only" checkbox makes J/K, Prev/Next, and Auto-advance
skip non-highlights too, so a day with thousands of detections plays
as a short reel of just the loudest events. Both key pairs also work
during full-file playback, and modified keypresses (Ctrl+K, Ctrl+U)
are no longer hijacked from the browser.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -40,7 +40,7 @@ Dependencies: `requests` (streams), `numpy` + `soundfile` (FLAC output and FLAC
 `webui.html` (one `<script>` block):
 - Clip review: `clipQueue`/`clipCursor` globals, `playClip()`, `playFileSection()`, `hideClipBar()`; markup is the `#clip-bar` div.
 - Day review: `dayHighlights()` builds `dayActiveSections` (chronological); `jumpToDaySection()` arms the queue.
-- J/K: single document-level `keydown` listener — clip queue takes priority, in-player `currentTime` stepping is the fallback when no queue is armed.
+- J/K/U/I: single document-level `keydown` listener — clip queue takes priority, in-player `currentTime` stepping is the fallback when no queue is armed. U/I (and the `#clip-hl-only` checkbox, which also affects J/K, Prev/Next, and auto-advance) restrict stepping to highlights: the top `#clip-top` (default 50) sections by score, computed on demand by `topScoreSet()`; `stepClip()` is the shared queue-stepping path.
 - Analysis: `fetchAnalysis()` (session `analysisCache`), `analyse()` (per-row render), `cachedParamsMatch()` (autoload guard).
 
 ## Verifying changes