feat: add MIDI trigger binding as an additional transcription input

[lucinadin]

Before Submitting This PR

Please confirm you have done the following:

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

If this is a feature or change that was previously closed/rejected:

• I have explained in the description below why this should be reconsidered
• I have gathered community feedback (link to discussion below)

Human Written Description

I added MIDI input support as an additional way to trigger transcription, without replacing or changing the existing keyboard shortcut behavior. I wanted this for convenience in my own workflow (MIDI pedal trigger while keyboard shortcuts are already used elsewhere), and implemented it to match existing settings patterns.

Testing

• bun run format:check
• bun run lint
• bun run build
• bun run check:translations
• cargo check
• cargo clippy (remaining warnings are pre-existing in unrelated files)
• Ran app in dev mode: bun run tauri dev
• Verified startup with previously selected but disconnected MIDI device
• Verified MIDI dropdown disabled when no devices are available
• Verified saved disconnected device is shown in selector as disabled
• Verified bind button is disabled unless selected device is currently available
• Verified binding flow works after reconnecting device
• Verified trigger reset works
• Verified existing keyboard shortcut flow still works

I only tested on macOS (Mac mini M4 Pro / Silicon). I tried the MIDI flow with 3 different MIDI devices (selection, binding, reset, trigger behavior)

AI Assistance

• No AI was used in this PR

• AI was used (please describe below)

• Tools used: OpenCode (gpt-5.3-codex)

• How extensively: Implementation/refactoring support across Rust and React, i18n, command/event wiring, validation runs, and iterative bug fixing.

[cjpais]

Right now new features are not being accepted. You can read the stipulations for this if you really want this feature. They were there when you submitted the pull request and you did not address them.

[lucinadin]

Sorry about that! I really like the app and actually built this locally for my own setup first. I completely missed the feature freeze note until after I opened the PR, but I figured I'd share it in case it could be useful to others too. I'll keep using my local build for now, but let me know when you're accepting new features again and want to revisit it.