Implement `subminer stats -b` to start a background stats daemon and
`subminer stats -s` to stop it, with PID-based process lifecycle
management, single-instance lock bypass for daemon mode, and automatic
reuse of running daemon instances.
- Add secondary_text column to imm_subtitle_lines with migration
- Pass currentSecondarySubText through recordSubtitleLine flow
- Include secondaryText and sourcePath in word/kanji occurrence queries
- Update all type interfaces (backend + frontend)
- Stats server auto-start on immersion tracker init
- Stats overlay toggle via keybinding and IPC
- Stats CLI command (subminer stats) with cleanup mode
- mpv plugin menu integration for stats toggle
- CLI args for --stats, --stats-cleanup, --stats-response-path
- show a dedicated startup OSD "building" phase for character dictionary sync
- forward bare `Tab` from visible overlay to mpv so AniSkip works while focused
- fix Windows plugin env override resolution for `SUBMINER_BINARY_PATH`
- Add default `f` fullscreen overlay binding and switch default AniSkip skip key to `Tab`
- Make character-dictionary auto-sync non-blocking at startup with tokenization gating for Yomitan mutations
- Add ordered startup OSD progress (checking/generating/updating/importing), refresh current subtitle on sync completion, and extend regression tests
- move JLPT/frequency dictionary init off sync fs APIs and add cooperative yielding during entry processing
- decouple first tokenization from full warmup by gating only on Yomitan readiness while MeCab/dictionary warmups continue in parallel
- update mpv pause-until-ready OSD copy to tokenization-focused wording and refresh gate regression assertions
