fix: Audio-to-MIDI conversion should respect clip trim/offset and validate confidence display end-to-end

[ChuxiJ]

Problem

Audio-to-MIDI conversion appears to use the full audio key and BPM without passing clip trim or audioOffset. Confidence rendering is unit-tested, but there is no E2E coverage for convert -> piano roll confidence display.

Root Cause

Conversion service integration was added before trimmed clip semantics and confidence visualization were validated together in a browser workflow.

Solution

• Pass clip trim/start/end/audioOffset into transcription or pre-slice the audio sent for conversion.
• Preserve timing alignment between the converted MIDI and the clip region.
• Add E2E coverage for conversion and confidence visualization in the piano roll.

Verification

• Add unit tests for trimmed clip conversion input ranges.
• Add E2E or integration test for an offset clip converted to MIDI with confidence colors visible.
• Run focused audio-to-MIDI tests.

Files to Touch

• src/store/projectStore.ts
• src/services/audioToMidi.ts
• src/components/pianoroll/PianoRollRenderer.ts
• tests/e2e/*audio-to-midi*