Add microphone cutoff option

[sandstorm244]

Content

Adds a user-configurable microphone cutoff volume (noise gate): mic input quieter than a chosen dBFS threshold is muted before being published.

• New gate stage in the existing RNNoise AudioWorklet (level-based, with hysteresis, ~300ms hold and open/close gain smoothing) — works with RNNoise enabled or disabled
• New Audio settings section: enable checkbox + cutoff slider (−60…−10 dB, default −40 dB)
• The slider doubles as a live input meter fed by the worklet's own level measurement — green while above the cutoff (transmitting), grey while below (muted) — so the meter and gate are calibrated identically
• Threshold changes apply live via worklet messages; no track restart needed. Gate-only mode lifts the 48kHz sample-rate requirement (that's RNNoise-specific)

Motivation and context

Because noise supression just isnt enough, a cut-off helps quite alot.
Similar PR upstream got closed for some reason perhaps we can have this nice feature here?

Screenshots / GIFs

Tests

• 16 new unit tests: gate DSP through the worklet test harness (mutes below cutoff, passes above, hold window, reopen timing, level reporting), processor config plumbing, Publisher attach/detach/live-update, settings UI
• Manually against the local devenv backend: two participants, one muted + listening; tuned the cutoff with the live meter, confirmed speech cuts out above threshold, background noise (tapping on a can lol/keyboard) muted at −40 dB, word endings not clipped
• Verified with RNNoise on and off, and that disabling the cutoff removes the processor entirely
• lint:types, lint:oxlint, lint:knip, format:check, i18n:check, build:full, build:embedded all pass

For transparency sake, i have used AI in this PR.

Feel free to adjust or close if there is a better implementation.