Say when there is no microphone rather than showing one at rest

With no microphone attached the menu drew a level indicator resting at
zero, announcing "No sound detected" — which is exactly what a working
but silent microphone draws. The two states people most need to tell
apart looked the same.

The level state now distinguishes a microphone that is absent from one
that is merely quiet, and the microphone group names the absence where
the indicator would be, as it already does for a denied permission. A
microphone that is merely slow to open still draws at rest, so the
indicator is on screen as soon as the menu is.

The product spec listed this as an edge case but no requirement carried
it, so no acceptance criterion covered it either. Added as AC28 with the
owner's authorisation, and the product spec's edge case reworded to say
what the menu does rather than that it shows nothing.

Spec: FEATURES_SPEC/2026-09_Audio_Quick_Menu.md — AC28.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

locales/en/app.json

@@ -24,6 +24,7 @@
   },
   "analytics_notice": "By participating in this beta, you consent to the collection of anonymous data, which we use to improve the product. You can find more information about which data we track in our <2>Privacy Policy</2> and our <6>Cookie Policy</6>.",
   "audio_menu": {
+    "mic_absent": "No microphone found",
     "mic_level_active": "Picking up sound",
     "mic_level_label": "Microphone level",
     "mic_level_silent": "No sound detected",