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/lib/platform.ts

@@ -33,3 +33,17 @@ export const isMacActionFallback = (
 /** Match action-modifier + a single character (case-insensitive). */
 export const isAction = (key: { ctrl: boolean; meta: boolean; super?: boolean }, ch: string, target: string): boolean =>
   isActionMod(key) && ch.toLowerCase() === target
+
+/**
+ * Voice recording toggle key (Ctrl+B).
+ *
+ * Documented as "Ctrl+B" everywhere: tips.py, config.yaml's voice.record_key
+ * default, and the Python CLI prompt_toolkit handler. We accept raw Ctrl+B on
+ * every platform so the TUI matches those docs. On macOS we additionally
+ * accept Cmd+B (the platform action modifier) so existing macOS muscle memory
+ * keeps working.
+ */
+export const isVoiceToggleKey = (
+  key: { ctrl: boolean; meta: boolean; super?: boolean },
+  ch: string
+): boolean => (key.ctrl || isActionMod(key)) && ch.toLowerCase() === 'b'