feat(transcription): expose native punctuation ITN

[LauraGPT]

Summary

• add a persisted native punctuation and inverse text normalization (ITN) toggle in model settings
• send Itn::On / Itn::Off to batch and live transcribe-cpp only when the loaded model advertises Feature::Itn
• apply the same persisted preference to SenseVoice use_itn, so its visible toggle is functional rather than fixed-on
• default the setting to enabled, preserving existing default behavior

Unsupported transcribe-cpp model families retain Itn::Default; no unsupported request fields are added.

Addresses #2015.

Verification

• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• cargo test --manifest-path src-tauri/Cargo.toml itn_mode --lib
• git diff --check

The focused Rust tests cover enabled and disabled choices for both capability-gated transcribe-cpp models and SenseVoice. The latest commit is SSH-signed and DCO signed-off.