fix(tui): route Ctrl+B to voice toggle, not composer input

When the user runs /voice and then presses Ctrl+B in the TUI, three
handlers collaborate to consume the chord and none of them dispatch
voice.record:

- isAction() is platform-aware — on macOS it requires Cmd (meta/super),
  so Ctrl+B fails the match in useInputHandlers and never triggers
  voiceStart/voiceStop.
- TextInput's Ctrl+B pass-through list doesn't include 'b', so the
  keystroke falls through to the wordMod backward-word branch on Linux
  and to the printable-char insertion branch on macOS — the latter is
  exactly what timmie reported ("enters a b into the tui").
- /voice emits "voice: on" with no hint, so the user has no way to
  know Ctrl+B is the recording toggle.

Introduces isVoiceToggleKey(key, ch) in lib/platform.ts that matches
raw Ctrl+B on every platform (mirrors tips.py and config.yaml's
voice.record_key default) and additionally accepts Cmd+B on macOS so
existing muscle memory keeps working. Wires it into useInputHandlers,
adds Ctrl+B to TextInput's pass-through list so the global handler
actually receives the chord, and appends "press Ctrl+B to record" to
the /voice on message.

Empirically verified with hermes --tui: Ctrl+B no longer leaks 'b'
into the composer and now dispatches the voice.record RPC (the
downstream ImportError for hermes_cli.voice is a separate upstream
bug — follow-up patch).

ui-tui/src/app/useInputHandlers.ts

@@ -8,7 +8,7 @@ import type {
   SudoRespondResponse,
   VoiceRecordResponse
 } from '../gatewayTypes.js'
-import { isAction, isMac } from '../lib/platform.js'
+import { isAction, isMac, isVoiceToggleKey } from '../lib/platform.js'
 
 import { getInputSelection } from './inputSelectionStore.js'
 import type { InputHandlerContext, InputHandlerResult } from './interfaces.js'
@@ -370,7 +370,7 @@ export function useInputHandlers(ctx: InputHandlerContext): InputHandlerResult {
       return actions.newSession()
     }
 
-    if (isAction(key, ch, 'b')) {
+    if (isVoiceToggleKey(key, ch)) {
       return voice.recording ? voiceStop() : voiceStart()
     }