feat: duration and seeking for in-progress FLAC recordings

FLAC duration cannot be derived from byte size (variable compression),
so unlike WAV the header cannot be patched from st_size alone. Instead,
every FLAC frame header carries its own frame/sample number: read the
last 64 KB of the growing file, scan backwards for a frame sync,
CRC-8-verify the header to reject false matches in compressed data,
and compute the exact samples recorded so far. STREAMINFO
total_samples (36 bits at a fixed offset) is rewritten in the served
bytes only - the on-disk file is never touched.

Overhead: one tail read per /stream request, active files only.

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

README.md

@@ -169,7 +169,7 @@ Shows recordings grouped by day with collapsible sections. Features:
 - **Filters** — live filename search and from/to date pickers above the table; applied client-side with no additional requests. Shows `N of M shown` when a filter is active.
 - **Delete** — `✕ Delete` button per row with confirmation prompt; disabled for files currently being recorded; sends `DELETE /api/files/<name>` and re-renders the table.
 - **Live REC badge** — files currently being written by `isr.py` show an animated REC indicator, polled every 5 seconds via `/api/status`. Duration for in-progress files shows `—` in the table (header is unfinalized until recording stops). The file list refreshes automatically when a recording starts, stops, or rolls over to a new split file (unless audio is playing).
-- **Listen while recording** — in-progress files are playable and seekable. For WAV the server patches the (still unfinalized) header on the fly so the browser sees the real duration-so-far; reopening the player reloads the source to pick up newly recorded audio. Live responses are sent with `Cache-Control: no-store`.
+- **Listen while recording** — in-progress files are playable and seekable. For WAV and FLAC the server patches the (still unfinalized) header on the fly so the browser sees the real duration-so-far — for FLAC the exact sample count is parsed from the last frame header in the file tail. Reopening the player reloads the source to pick up newly recorded audio. Live responses are sent with `Cache-Control: no-store`.
 - **Fast loading** — analysis results are cached server-side on disk and client-side per session; cached waveforms load only for expanded day groups, and collapsed days fetch nothing until opened.
 - **WCAG-compliant** — skip link, `aria-expanded`/`aria-controls` on the player toggle, `aria-live` status, focus management, `role=img` on SVG waveforms.