feat: import WAV files as audio clips

[britle20]

Summary

Implements WAV file import as audio clips from the sidebar clip add flow, including selected-clip loop preview playback.

Closes #39.

Changes

• Added Build a clip / Import a file menu to the sidebar clip add button.
• Added WAV file validation, imported audio clip metadata helpers, and unit tests.
• Added AudioClip / SampleMeta model types and made the app clip collection support hybrid and audio clips.
• Added audio-engine runtime decoding/cache support for imported files without storing File, Blob, object URLs, or AudioBuffer in serializable clip data.
• Added imported audio clip detail view with duration, source file, sample ID, session-only persistence warning, and loop preview/stop controls.
• Added audio-engine cached sample loop preview playback for imported audio clips.
• Preserved existing hybrid clip creation and editor behavior under Build a clip.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present
• git diff --check
• Dev server smoke check with curl -I http://127.0.0.1:5174/

Screenshots

Not captured in this environment.

Notes / Limitations

• Imported WAV clips are session-only. Refreshing the page loses runtime decoded audio until IndexedDB persistence is implemented.
• Preview playback loops from the runtime cache until stopped; arrangement placement/playback is still deferred to Arrangement clip placement and playback #41.
• Arrangement duration resizing, time stretching, and project export/import for imported file bytes remain deferred.

Follow-up

• Arrangement clip placement and playback #41 should use the new AudioClip shape when adding arrangement clip placement/playback.