feat: add per-app sound mixer with menu bar access

[lou1s19]

What

Adds a sound mixer to OnlySwitch — a new App Sound settings section and
an optional menu bar item that opens a compact mixer modeled on the macOS
system Sound panel:

• System volume slider
• A per-app row for every app currently playing audio, with mute and a
  volume slider
• The current output device

How it works

Two mechanisms, chosen automatically per app:

1. Scriptable media apps (Music, Spotify, TV, iTunes) — read and written
   through AppleScript's standard sound volume (0–100).
2. Any other app that outputs audio, including browsers like Chrome — via
   a Core Audio process tap (AudioHardwareCreateProcessTap, macOS 14.4+).
   macOS exposes no per-app output-level API, so the tap routes the app's audio
   through a private aggregate device and scales the samples itself (continuous
   gain, of which mute is the 0 case). It falls back to silencing on any
   non-Float32 stream format. Below macOS 14.4 only the scriptable apps appear.

Menu bar item

Opt-in via the App Sound settings toggle. It gets its own status item
(speaker.wave.2.fill), created before the hide-menu-bar "mark" item so that
collapsing the menu bar doesn't strip it. A hint in settings points to
System Settings › Control Center › Sound for anyone who wants it to stand in
for the system sound item.

Notes

• Adds NSAudioCaptureUsageDescription — the process tap triggers the system
  audio-capture permission prompt on first use.
• All new UI strings are added to Localizable.xcstrings in the full set of
  languages the project already ships.
• Manually tested on Apple Silicon: the App Sound settings page, the menu bar
  toggle (add/remove), and the mixer popover. Feedback welcome on the
  process-tap path across a wider range of apps.

Screenshots