fix(settings): persist activation mode across restarts

[hl9020]

Problem

Activation mode is the only setting on the App settings screen that does not survive a restart. Selecting "Toggle", quitting and relaunching brings the app back in "Hold" mode.

The reason is that activationMode only ever existed in React state. There is no field for it in Config, and no save_/get_ command pair, unlike hotkey, languages, endpointing, copy_to_clipboard and audio_device_selection. On every launch the value falls back to the hardcoded push-to-talk default in App.tsx.

Approach

Store it the same way the neighbouring settings are stored, so nothing new is introduced:

• activation_mode: Option<String> in Config, defaulting to push-to-talk, and covered by fill_defaults so existing config.json files migrate silently.
• save_activation_mode / get_activation_mode commands, with the setter rejecting anything other than the two valid values.
• The frontend loads the stored value during init and writes it back from an effect, guarded by a loaded ref exactly like copyToClipboard.

No change to hotkey handling or dictation behaviour - only where the already-existing value is read from and written to.

Testing

Windows 11, installed NSIS build. Set the mode to Toggle, quit, relaunch: the setting is retained and hold-to-talk no longer reactivates. Verified the same for switching back to Hold, and that an existing config.json without the field starts up on the previous default instead of erroring.

• npm test: 119 Rust tests, 26 vitest tests, typecheck all pass
• npm run build: clean
• npm run format: no changes to the files in this PR
• cargo clippy --all-targets -- -D warnings: fails identically on this branch and on unmodified main (22 pre-existing never used errors on Windows, since much of the accessibility code is macOS-only). No new warnings introduced.

Not verified on macOS - I have no Mac available. The change is platform-independent, but a second pair of eyes there would be welcome.

[karamouche]

Thanks for the contribution, this looks good to me!

One small ask so CI can pass: the commitlint check is failing on body-max-line-length (body lines must be ≤ 100 characters). The subject is fine; it’s the body paragraph that’s too long.

Could you amend/reword the commit message body so each line stays under 100 chars? Thanks

[hl9020]

Good catch, thanks - amended and force-pushed, body lines are now well under 100 characters.