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/components/textInput.tsx

@@ -623,7 +623,19 @@ export function TextInput({
         return
       }
 
-      if ((k.ctrl && inp === 'c') || k.tab || (k.shift && k.tab) || k.pageUp || k.pageDown || k.escape) {
+      // Ctrl+B is the documented voice-recording toggle (see platform.ts →
+      // isVoiceToggleKey). Pass it through so the app-level handler in
+      // useInputHandlers receives it instead of being swallowed here as
+      // either backward-word nav (line below) or a literal 'b' insertion.
+      if (
+        (k.ctrl && inp === 'c') ||
+        (k.ctrl && inp === 'b') ||
+        k.tab ||
+        (k.shift && k.tab) ||
+        k.pageUp ||
+        k.pageDown ||
+        k.escape
+      ) {
         return
       }