Add safe local song import and metadata truth UI

[Baltsat]

What this adds

• a prepared-song import flow: choose a Clone Hero folder, validate chart/audio/drum difficulties, review metadata and artwork, then add it to the selected library
• cover ingestion that preserves an existing manual cover, extracts embedded artwork, or accepts an explicit HTTPS image URL when the user has permission to cache it
• separate Auto-charted with STRUM provenance instead of displaying the AI engine as a human charter
• clear score-star semantics: unplayed charts say play once to earn stars; played charts expose their best accuracy to assistive technology and in a tooltip
• normalized local search across name, artist, album, charter, and auto-chart tool
• normalized exact-field ranking for Enchor results, plus a clear fuzzy-only notice
• a short settings hint showing where the installed stem splitter is actually used

Safety and compatibility

• imports prepared local chart folders only; there is no streaming-service or YouTube media extraction
• existing library folders are never overwritten or removed on a name collision
• source folders inside the library, source folders containing the library, and symbolic links are rejected
• imported AI provenance is normalized in the copied song.ini; the source folder is not changed
• manual album art is never overwritten
• remote artwork is optional, HTTPS-only, image-typed, size-limited, and framed as owned/permitted artwork

Proof

• yarn test — 58 files, 705 tests passed
• yarn lint — passed
• yarn test:e2e — production build plus 3/3 Electron/Playwright tests passed
• the E2E flow validates a prepared auto-chart, imports it, confirms the .sightkick sidecar and cover, searches it by STRUM, and verifies the provenance and score copy

The two commits are intentionally reviewable in order: the filesystem/IPC foundation first, then the renderer UX and E2E coverage.

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[tonygoldcrest]

Hey! Thanks for the PR.
It seems that it aims to implement multiple features.

Please, split it into multiple per-feature PRs so I have an easier time reviewing and testing them.

[Baltsat]

Split into the two independently reviewable PRs requested:

• feat: import prepared local songs #43 — prepared local-song import
• feat: normalize local song search #44 — normalized local metadata search

Both replacements target main directly, have one coherent feature each, and include their own tests and E2E proof. Closing this combined PR to avoid duplicate review.