Fix audio sync + a batch of correctness/robustness bugs

[xn101de]

A batch of independent, self-contained fixes (one commit each, so they can be
reviewed/cherry-picked individually).

Audio correctness

• Time-sync median sorted as strings — TimeProvider.setDiff sorted the
  clock-offset buffer with a bare Array.sort(), ordering numbers
  lexicographically → wrong median, unstable serverTime(), playback drift and
  Chunk too old, dropping stalls. Now sorts numerically. (likely Snapweb slightly out of sync with snapdroid clients  #39;
  contributes to Safari not working #49, Change snapweb client to stream in playback, does not work  #108)
• PCM normalization off by one power of two — divided by 2^bits instead of
  2^(bits-1), so all audio played ~6 dB too quiet.
• JSON message size used UTF-16 length — JsonMessage.serialize() wrote the
  string length while the buffer was sized from UTF-8 bytes, truncating payloads
  with non-ASCII characters (e.g. "Küche").

Robustness / React

• uuidv4() prefers crypto.randomUUID() with a safe fallback.
• prefers-color-scheme listener moved from render body into a useEffect with
  cleanup (was leaking a listener per render).
• Force-update counters use functional setState.
• Group volume guarded against divide-by-zero (NaN) for empty groups.

Cleanup

• Removed dead clamp expressions in MediaSession seek handlers.
• Removed a multi-KB commented-out sample server JSON.

Verified: tsc clean, vite build succeeds.

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