feat: add realtime whisper with OSD preview

[goodroot]

Enables the new OpenAI real-time whisper endpoint. And incorporates the deltas into the visualizer. So that you can see your voice transcribe in real-time prior to submitting.

[github-actions]

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Note: this review is partial because the PR diff was truncated to 120000 bytes from 245558 bytes.

Summary

Partial review only: the provided diff is truncated at 120000 of 245558 bytes, so this does not cover the full PR. Within the visible changes, I found a likely startup crash in mic-osd initialization and a regression where live preview text can be dropped whenever mic-osd is disabled/unavailable even though the realtime backend still emits partials.

Findings

• lib/mic_osd/main.py: MicOSD.__init__() unconditionally does VISUALIZATIONS["waveform"] as the default fallback. In this PR, failed imports set VISUALIZATIONS = {}; that makes startup raise KeyError before main() can return the intended “Unavailable” message. This turns missing GTK/PyCairo deps into a hard crash instead of graceful degradation. Guard the fallback before indexing, or short-circuit MicOSD construction when _MIC_OSD_IMPORT_ERROR is set.
• lib/main.py: realtime partial callbacks are always wired via self.whisper_manager.set_realtime_partial_callback(self._set_mic_osd_preview_text), but _set_mic_osd_preview_text() is a no-op unless self._mic_osd_runner exists. That means if mic-osd is disabled/unavailable, partial transcript events are silently discarded. If the websocket client expects the callback to forward or retain partials for any other consumer (capture mode, future UI, logs/tests), this is a behavior regression tied to OSD availability. The callback should either be registered only when preview is supported, or write through an abstraction that is independent of OSD enablement.

Suggested follow-ups

• Add tests for mic-osd unavailable/missing-dependency startup to verify it exits cleanly without KeyError.
• Add tests covering realtime partial preview lifecycle: start recording, partial updates, stop/cancel/error cleanup, and behavior when mic_osd_enabled=false.
• Review the omitted half of the diff before merge; the largest risk here is unreviewed websocket/realtime transcription changes outside the visible OSD plumbing.