feat: split the web UI into modules and rework it for screen readers

webui.html becomes webui/ (index.html, style.css, ES modules in js/, no
build step); web.py serves it under / and /ui/<path> with the same
containment check as recordings.

How it feels with a screen reader:
- Days read "Today, Saturday 17 October"; recordings are named by their
  wall-clock span ("22:00 to 23:00"), which is the row header and what
  every button says. The filename moves to the last column.
- Focus is never dropped: busy buttons stay focused (aria-disabled),
  finished analyses focus their first section, delete moves to the next
  row, closing the clip bar (x or Escape) returns to where you were.
- Reloads rebuild only the days whose content changed; DOM ids are stable
  per filename, so a rollover no longer resets the reading position.
- Only user-initiated actions are announced; highlights completion is
  spoken (the progress area was stuck aria-busy before).
- Media Session previous/next track step sections like J/K (AirPods,
  lock screen, media keys).
- Row layout: Time | Length | Loudness | Player | Options | File. The
  player opens inside its cell; Download/Cut/Delete are always there,
  and Cut toggles its start/end fields in place.
- Settings tucked behind a <details>; pre-roll and auto-advance are
  remembered per browser; no CSS uppercase on headers or badges.

Fixes the REC badge showing on every row: its inline-flex display beat
the UA [hidden] rule. One global [hidden]{display:none !important} now
covers every case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -10,7 +10,8 @@ WORKDIR /app
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
-COPY isr.py web.py webui.html ./
+COPY isr.py web.py ./
+COPY webui/ ./webui/
 COPY isrweb/ ./isrweb/
 
 RUN mkdir -p /recordings