fix: reject silent microphone captures in clamshell mode

[jatinkrmalik]

Summary

• Stop effectively silent microphone captures before they reach Whisper, preventing subtitle-style hallucinations from disconnected input.
• Show an actionable message directing clamshell users to select an external microphone.
• Remove the duplicate-looking (System Default) suffix from physical devices in the microphone picker.

Root cause

Modern MacBooks hardware-disconnect the built-in microphone when the lid closes. macOS can continue reporting that microphone as the default input, so VocaMac receives a non-empty buffer containing digital silence. The existing empty-buffer check passes that audio to Whisper, which may generate phrases such as "thank you" or foreign-language subtitle text.

Explicitly selecting an external microphone works because VocaMac binds its AudioUnit directly to that Core Audio device.

User impact

VocaMac now rejects disconnected digital silence instead of inserting hallucinated text and tells the user how to recover. It does not silently switch microphones; users retain control over which device records audio.

Validation

• swift test
• 226 tests executed, 1 permission-dependent test skipped, 0 failures
• Added coverage confirming silent capture neither invokes Whisper nor injects text
• git diff --check