feat: add Xiaomi MiMo ASR and explicit hotkey trigger modes

[fish2lab]

Summary

• Add a native Xiaomi MiMo STT adapter using /chat/completions, Base64-encoded WAV input, and asr_options.language.
• Register ProviderKind::Mimo throughout provider validation, registry construction, migrations, and the settings preset list.
• Replace duration-based hotkey intent detection with an explicit Hold to talk / Press to toggle setting.
• Apply the selected trigger mode consistently to dictation, translation, and assistant sessions.
• Migrate settings schema from v10 to v11 while preserving existing users on hold.

Motivation

MiMo ASR does not use the multipart /audio/transcriptions wire format used by OpenAI-compatible STT providers. It requires a dedicated JSON request shape, so treating it as a generic OpenAI-compatible endpoint would produce an invalid request.

The previous hotkey state machine inferred hold versus toggle behavior from a 350 ms release boundary. This made short presses timing-sensitive and unreliable for page turners and third-party keyboards that emit brief down/up pulses instead of a stable held-key signal. Assistant toggle sessions also waited for the second release, creating an additional sticky interaction.

Behavior

• hold: the first keydown starts recording and the matching keyup stops it.
• toggle: the first keydown starts recording, releases are ignored, and the second keydown stops recording immediately.
• The trigger mode is snapshotted when recording starts, so changing settings does not alter an active session.
• Existing v10 settings migrate to hold; the obsolete hold_threshold_ms field is removed.
• Long-audio incremental transcription and live dictation are intentionally out of scope.

Validation

• cargo test --manifest-path src-tauri/Cargo.toml --no-default-features --lib orchestrator::session::tests — 32 passed
• Targeted v10 → v11 migration test — passed
• pnpm gen:ipc — passed
• pnpm test — 160 passed
• pnpm tauri build --debug --bundles app -- --no-default-features — passed
• Manual MiMo cloud smoke test returned a successful transcription; the credential remained in local runtime configuration and is not present in the repository