feat: remove decorative day-timeline SVG, linear highlights panel

The user is blind; the day activity timeline (and its hour labels) was
purely visual, non-interactive, and carried no information not already
in the chips/summary. The highlights panel is now linear text and
buttons in reading order: summary line ("N files analysed - M loud
sections"), key-hint note (now always shown, J/K/U/I is the primary
interface), chips toggle, chips.

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

README.md

@@ -154,7 +154,7 @@ The browser UI (HTML/CSS/JS) lives in `webui.html`, which `web.py` loads at star
 Shows recordings grouped by day with collapsible sections. Features:
 
 - **Day groups** — recordings are grouped under a collapsible day heading showing date, file count, total duration, and total size. The most recent day is expanded by default; older days start collapsed. Expanded state is preserved across filter changes.
-- **Day highlights** — click **Highlights** on any day heading to run loudness analysis across all WAV/FLAC files in that day and display a combined activity timeline SVG. The button is a toggle: clicking again collapses the panel, and re-expanding it reuses the already-computed results (they are only recomputed when the analysis parameters change). A **· analysed** suffix on the button marks days where every file already has a cached analysis for the current parameters, i.e. highlights open instantly. Orange segments show when loud sections occurred relative to the day's time span; blue shows the file extents. The timeline axis is labelled at round wall-clock hours, with matching tick marks on the bar. When a day has more sections than fit as chips, the chips are the top 50 by score (loudest-above-background first) so the most promising events are reviewed first; long chip lists are collapsed behind a toggle button so the panel stays compact. J/K still steps through all sections in time order, and U/I steps through them by loudness.
+- **Day highlights** — click **Highlights** on any day heading to run loudness analysis across all WAV/FLAC files in that day. The button is a toggle: clicking again collapses the panel, and re-expanding it reuses the already-computed results (they are only recomputed when the analysis parameters change). A **· analysed** suffix on the button marks days where every file already has a cached analysis for the current parameters, i.e. highlights open instantly. The panel is plain text and buttons in linear reading order (screen-reader friendly): files-analysed/section totals, a key hint, then the section chips. When a day has more sections than fit as chips, the chips are the top 50 by score (loudest-above-background first) so the most promising events are reviewed first; long chip lists are collapsed behind a toggle button so the panel stays compact. J/K still steps through all sections in time order, and U/I steps through them by loudness.
 - **Inline playback** — collapsible `Play` button per row; audio loads lazily via a seekable `/stream/` endpoint with HTTP Range support. Metadata is fetched immediately so the duration is visible without pressing play.
 - **Waveform analysis** — on demand per file; computes RMS per 100 ms window and marks sections that stand out above the background. Detection is **adaptive**: a rolling noise floor (20th percentile per 30 s block) is estimated across the file, and a section is flagged when the level rises at least *margin* dB (default 12) above that floor. Slow ambience changes — rain setting in, day/night traffic hum — move the floor instead of producing false positives. Each section gets a **score** used to rank it: its peak dB above the floor, capped by the sharpest rise within 0.5 s. Abrupt events — voices, impacts, barks — rise fast, so their score is their full prominence; a gradual swell (a gust, a distant approaching car) that drifts up faster than the floor can track still gets flagged, but scores near zero and sinks to the bottom of the highlight ranking. Supported for WAV and FLAC (FLAC requires `numpy` + `soundfile`). Pure-Python fallback for WAV when numpy is absent. Results are cached in `recordings/analyses/<filename>.analysis.json`; subsequent requests at the same margin, min-gap, and min-duration settings return instantly without re-reading the audio. The cache file is deleted automatically when the audio file is deleted. Orphaned cache files (audio deleted outside the UI) are pruned on startup.
 - **Grace period** — configurable in the controls bar (default 2 s). Loud sections separated by less than this gap are merged into one. Raise this (e.g. to 15–30 s) when a single event generates many timestamps due to brief quiet gaps within it.