Add microphone mute control

[pranc1ngpegasus]

What changed

• add a Mute mic / Unmute mic control while a recording is active
• route mute commands through the desktop session worker to the native audio session
• replace microphone input with silence while muted so system audio continues and both timelines stay aligned
• expose the mute operation through the Swift C ABI and safe Rust wrapper
• show the muted state in the recording status bar
• suppress microphone recognition results in the Windows preview backend

Why

Users need to temporarily exclude their microphone from a recording without stopping the session or interrupting system-audio capture and transcription.

Impact

During an active recording, users can mute and unmute the microphone. Muted intervals remain as silence in mic.ogg, preserving synchronization with system.ogg; microphone speech from those intervals is not transcribed.

Validation

• cargo test -p wisp-audiokit --all-targets
• cargo clippy -p wisp-audiokit --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• clean swift build --scratch-path /tmp/wisp-audiokit-mute-build

cargo check --workspace --all-targets reached the desktop GPUI dependency but could not complete because this machine does not have the Metal compiler (xcrun: unable to find utility "metal"). swift test is also unavailable in the installed Command Line Tools because the XCTest module is missing; the Swift package itself builds successfully.