feat: AI feature toggles in menubar dropdown

[mattthewong]

Summary

• Add 4 checkbox menu items under "AI Features" section in the menubar
• AI post-processing -- toggle Claude grammar cleanup
• Prompt mode -- toggle "summarize my clipboard" etc.
• Voice commands -- toggle "create PR", "git status" etc.
• Context-aware formatting -- toggle app-specific output
• Follows exact same pattern as existing Play sounds / Auto-paste toggles
• Each sends a bool on a Go channel for the settings watcher to consume

Menubar Layout (after this PR)

Status: Idle
---
Pause Vox
---
Hotkey: Option+Space
Change Hotkey       >
Mode                >
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Play sounds         [x]
Auto-paste          [x]
---
AI Features
AI post-processing  [ ]
Prompt mode         [ ]
Voice commands      [ ]
Context-aware       [ ]
---
Show Log...
---
Quit Vox

Moonshot

Part of Moonshots XXIII: Vox 2.0 -- P2: AI Feature Toggles in Menubar

Note: AI-generated via Claude Code. Review the Obj-C menu item lifecycle and Go channel wiring.

Test plan

• make build + make lint + make test-short all pass
• Manual: make start, verify AI Features section appears in menubar dropdown
• Manual: Toggle each checkbox, verify state persists across menu open/close

🤖 Generated with Claude Code

[alohaninja]

What we fixed

• Dead channels / non-functional checkboxes — Added 4 case arms in settingsWatcher (cmd/vox/main.go) so the Go side actually drains aiPostProcessCh, promptModeCh, voiceCommandsCh, and contextAwareCh. Clicks now flow through to runtime settings and persist to preferences.json.
• Checkboxes never visually toggled — Because settingsWatcher now calls ui.SetAIPostProcess(on) etc. on each toggle, the Obj-C round-trip completes and setState: fires. Checkboxes reflect the actual state.
• No startup sync — Added ui.SetAIPostProcess(settings.AIPostProcess.Load()) etc. after ui.Init(), seeded from flagClient.*(). Checkboxes now show the correct initial state from LD flags / env vars / config.
• No persistence — Added AIPostProcess, PromptMode, VoiceCommands, ContextAware *bool fields to config.Prefs. Each toggle persists via config.SavePref on click, survives restarts.
• No runtime effect — Updated all 4 pipeline stages (classifyStage, postProcessStage, promptModeStage, commandStage) to read settings.* atomic bools instead of flagClient.*(). Menubar toggles now actually enable/disable AI features at runtime.
• aiHeader memory nit — Added autorelease to the NSMenuItem alloc for the "AI Features" section header, matching the pattern used by showLogItem and quitItem.

What we didn't fix

• send() drops events on rapid toggling — The existing send() helper uses a non-blocking default branch on buffer-1 channels. If the user clicks faster than settingsWatcher can process (e.g., disk I/O in SavePref), intermediate clicks are silently dropped. This is a pre-existing design decision shared by all toggles (sounds, auto-paste, etc.) — not introduced by this PR. The last-written value is always correct; only intermediate transitions are lost.
• Repetitive boilerplate — 4 identical channel/setter/callback/handler patterns across Go and Obj-C. A table-driven or generic approach could reduce this but would be a refactor beyond this PR's scope.
• No unit tests for internal/ui/ — The cgo/AppKit nature of this code makes it impractical to unit test without mocking the Cocoa runtime. Pre-existing gap, not worsened by this PR.