feat(subtitles): add local Japanese subtitle generation (#240)

changes/japanese-subtitle-generation-docs.md

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+type: docs
+area: subtitles
+
+- Explain how dialogue generation retains uncertain audible sections, why songs may also be transcribed, how detected speech starts guide long-passage cuts to reduce early subtitles, and why each passage uses a fresh Whisper process.

changes/japanese-subtitle-generation.md

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+type: added
+area: subtitles
+
+- Generate local Japanese SRT subtitles with whisper.cpp from a standalone modal opened with Ctrl+Shift+G, the empty subtitle sidebar's generation button, or `subminer generate-subs`, with shared progress reporting, cancellation, safe output files, and automatic loading into the matching mpv video. The sidebar button hides while subtitle lines are loaded.
+- Configure an existing multilingual model in Settings or choose an official multilingual model, including quantized variants, in the modal or launcher. The modal shows download sizes, speed and accuracy guidance, and a recommended starting model before explicitly downloading a verified SubMiner-managed model. Executable paths are optional overrides; empty fields find whisper-cli, ffmpeg, and ffprobe on PATH. The modal's Local tools check and the launcher name any missing executable and its setting before downloading a model or extracting audio, and generation confirms the destination directory grants write and search permissions up front.
+- Optionally select Focus on spoken dialogue in the modal and use Download speech detection model to install the separate Silero model with progress and cancellation. The choice lasts for the session; a configured VAD model path sets the default. Retain uncertain audible sections so VAD rejection does not discard dialogue under music, accepting that songs may also be transcribed. Keep passages intact within Whisper's audio window, split longer passages near detected speech starts or quiet pauses with overlapping context to reduce early subtitle timing, and combine overlapping duplicate cues even when punctuation differs. Run each passage in a fresh Whisper process to prevent repeated-character output caused by state carried between files, at the cost of reloading the model per passage. Preserve original media timing and separate repeated dialogue.

changes/subtitle-sidebar-gap-follow.md

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+type: fixed
+area: subtitles
+
+- Keep the subtitle sidebar near playback during gaps when the subtitle file has a cue starting at zero.