feat: normalize local song search

[Baltsat]

Summary

• normalize local search text with Unicode decomposition, diacritic folding, case folding, whitespace normalization, and trimming
• search name, artist, album, and charter metadata
• rank exact, prefix, and substring matches ahead of fuzzy matches while preserving stable source order

This is the search portion of #42, split into a standalone PR at the maintainer's request. It applies directly to main and does not depend on the import feature.

Proof

• 46 focused search and view tests passed
• full suite passed: 56 files / 685 tests
• lint passed
• TypeScript typecheck passed
• application build passed
• 3/3 Electron E2E tests passed, including normalized album and charter queries
• production-hunk revert made the focused regression fail; restoring the final tree returned it to 46/46

AI assistance: I used Codex during implementation and test iteration, then reviewed the final diff and proof.

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[tonygoldcrest]

LGTM. I'll merge after releasing the next version (probably tomorrow).