Show a live microphone level in the audio menu

- Draw the input level beneath the microphone list as 32 discrete segments,
  so the level reads by count and not by hue alone.
- Expose it through role="meter" with aria-valuenow and aria-valuetext, so a
  screen reader announces it.
- Give denied permission and no input device their own message and next
  action, rather than a flat meter that reads as silence.
- Read the level only while the menu is open, so nothing holds a second
  capture of the microphone for the length of a call.
- Read the device the picker has selected, so the meter re-points as soon as
  the selection changes.
- Ignore anything below a noise floor: a quiet room is never digitally
  silent, and its hiss otherwise lights the first segments permanently, which
  reads as "it can hear me" when nobody is speaking.
- Quantise loudness with a square root above that floor: amplitude puts
  ordinary speech near the bottom of the range, where a linear meter barely
  moves.
- Withhold the output select callback where the platform cannot route audio
  to a chosen device, which is what renders the speaker section disabled.
  Safari and most Firefox builds land here.
- Track the in-flight device by kind as well as id: Chrome names both an
  input and an output "default", so the spinner appeared on the wrong row.
- Disable every device in the menu while a selection is settling, so a second
  request cannot overtake the first.

The meter opens its own short-lived capture rather than tapping the call's
audio track. That is what lets it follow the picker instantly, and it avoids
the pre-join track, which is deliberately frozen to the device selected when
the screen mounted. The cost is a second capture of the same device while the
menu is open. See the spec's notes sidecar.

Spec: FEATURES_SPEC/2026-09_Quick_Audio_Menu.md — AC1, AC6, AC7, AC13, AC14,
AC15, AC17, AC18, AC21, AC22

locales/en/app.json

@@ -163,6 +163,12 @@
   "login_auth_links_prompt": "Not registered yet?",
   "login_subheading": "To continue to Element",
   "login_title": "Login",
+  "microphone_level": {
+    "label": "Microphone level",
+    "no_device": "No microphone found. Connect one, then try again.",
+    "permission_denied": "Element Call cannot use your microphone. Allow access in your browser settings.",
+    "value": "{{level}} of {{max}}"
+  },
   "microphone_off": "Microphone off",
   "microphone_on": "Microphone on",
   "mute_microphone_button_label": "Mute microphone",