feat: add opt-in always-toggle shortcut alongside push-to-talk

[ljden]

Before Submitting This PR

Please confirm you have done the following:

• [X ] I have searched existing issues and pull
  requests (including closed ones) to ensure this isn't a duplicate
• [X ] I have read CONTRIBUTING.md

Human Written Description

I just downloaded Handy today and after using it for a few hours it was the first feature I noticed that was missing.
There's definitely times where I want to be able to just quickly pop something in and other times where a longer bit of text is needed and therefore having the ability to do both is good.

The main thing that I didn't do here that I would have preferred to do is to change the push-to-talk pill control from the current toggle | PTT behaviour, to instead be disabled | enabled, with the separate control for the toggle feature.
The one other thing that I didn't do was make the default binding as unset - as it seems like that's not currently supported. That becomes more useful with the independent controls.

I did see the discussion early in the year around the fact that you weren't sure about adding this at the time, so fair enough if you want to reject it!

Related Issues/Discussions

Related to #147 — implements the two-shortcut variant proposed in the comments
(a separate toggle shortcut alongside push-to-talk), rather than the
press-vs-hold hybrid in the original post.

Design decisions worth flagging:

• Opt-in and off by default: a new transcribe_toggle_enabled setting gates
  registration exactly the way post_process_enabled gates the post-process
  shortcut, so existing users see zero change (the frozen v0.9 settings-store
  test still passes; the new field is #[serde(default)], no migration).
• With Push To Talk off, the main transcribe shortcut already toggles, so
  enabling this one is redundant (though harmless). It is deliberately not
  gated on push_to_talk — that would couple registration to two settings and
  create hidden-active-shortcut edge cases for a cosmetic gain. Happy to add
  that gating as a follow-up if preferred; per-binding modes seem better left
  to the keyboard rewrite mentioned in Auto push to talk and command line options #147.
• The Cancel shortcut row is now shown whenever any shortcut can start a
  toggle-mode recording (previously hidden whenever Push To Talk was on).

Community Feedback

In #147, @cjpais said "I like the feature you're describing", and @codepunkt /
@samleibowitz specifically asked for "defining two shortcuts, one for push to
talk and one for toggle" — which is what this PR adds.

Testing

• cargo check, cargo clippy, cargo fmt --check clean (no new warnings)
• cargo test: 177 passed, including the frozen v0.9 settings-store
  compatibility test
• bun run lint, tsc, prettier --check, vite build all pass
• Manual on macOS: tested with new toggle both enabled and disables to confirm correct behaviour. Also tested both states of PTT with the new explicit toggle key combo.

AI Assistance

• No AI was used in this PR
• AI was used (please describe below)

If AI was used:

• Tools used: Claude Code (Sonnet 5 wrote the initial implementation; Fable 5 reviewed and reworked it)
• How extensively: code and commit message AI-written under human direction; human reviewed the diff, ran all
  checks, and manually tested the feature

[adamkdean]

Cross-posting for visibility: #1878 (comment)

My preference would be that we have one hot key, and it's 200-300ms key up/down for toggle, or longer for PTT. This would be the best UX.