feat: add configurable maximum dictation duration

[NathanSkene]

Summary

• Adds a configurable maximum recording duration setting
• Recording automatically stops when the limit is reached
• Prevents accidentally long recordings that may fail transcription

Test plan

• Set max duration in Settings
• Start recording and wait for limit
• Verify recording stops automatically at the configured duration
• Verify transcription processes normally after auto-stop

[NathanSkene]

Manual validation update on macOS with the packaged installed app:

This feature passed validation after fixing a gap in the original implementation.

What I verified in the real installed app:

• setting a max dictation duration causes recording to stop automatically at the configured limit
• transcription still completes normally after the auto-stop

Important implementation note:

• the original branch did not wire max-duration enforcement into the real hotkey/floating recorder path, only into the notes widget path
• I patched the real recorder path locally during validation and confirmed the intended behavior there

So the user-facing feature is valid, but the working version is the one with the recorder-path fix included.