fix(player): route recordings by source extension, not content sniff (#89)

[indigo423]

Fixes the core of #89: Library and Local playback fails for tracker modules ("Couldn't play that track — pick another"); only Mod Archive works.

Root cause

AudioPlayer.play() content-sniffed every buffer via mimeForBuffer to detect PCM recordings. Its deep MP3 frame-sync scan (deepScanMp3 — scans 64 KB for ≥3 0xFF-framesync byte patterns) false-positives on a tracker module's raw 8-bit PCM sample data. So a genuine .mod/.xm/etc. was classified as audio/mpeg, routed to the <audio> element (which can't decode a module → MEDIA_ERR, [pcm] 4), and failed.

Why only Mod Archive worked: its play-random silently skips a failing track and advances, masking the bug; a user-picked Library/Local row stays failed.

Confirmed live on prod (v0.5.11) and reproduced in the current build — mimeForBuffer(4mat_VI.mod) returns "audio/mpeg".

Fix — trust the source, don't sniff module bytes

Recordings only ever arrive from Library/Local sources with a recording extension (Mod Archive is always a module; AHX is caught by reliable offset-0 magic; TFMX by object shape). So classify recordings by the source's extension and stop feeding arbitrary module buffers into the MP3 sniffer:

• recording-magic.ts: new mimeForExtension(name); export MimeType.
• sources/index.ts: recordingMime(source) — MIME from Library path / Local filename, null for Mod Archive/TFMX.
• audio-player.ts: play(input, pcmMime?) takes the caller's recording hint; the PCM arm fires on pcmMime instead of mimeForBuffer(input).
• Player.tsx + EmbedPlayer.tsx: pass recordingMime(source).

mimeForBuffer is unchanged and still used for download-time extension guessing (Mod Archive has no filename).

Verification

Live (local dev against real files pulled from prod):

• A real Library .mod now plays through libopenmpt ("Playing from: library", no error).
• An .mp3 recording still routes to the PCM engine and plays.
• New unit tests for mimeForExtension and recordingMime (incl. .mod → null); 222 passing; typecheck + lint clean.

Out of scope (separate issues from #89)

• Some Misc/ChipTune files on prod are 0 bytes on disk — a data/sync issue on the server, not a code bug (/api/library/file faithfully streams stat.size). Worth a re-sync.
• mschwendt's original TFMX failure: those bytes serve fine and route before the sniffer, so it's unrelated to this bug — likely the AudioWorklet TextDecoder gap; needs its own confirmation from the browser console.

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