UX polish and import feedback improvements

[YapWH1208]

Summary

UX polish pass and Import/Library feedback improvements (P1 + P2):

• Import feedback: progress feedback for library operations, monotonic multi-stage progress, encode ETA + output size + duplicate-name warning, representative non-black preview frame
• Library UX: accept video drops on Library and follow the source when naming, highlight newly imported wallpaper, Restore Latest Backup (with clarified removal recovery)
• Robustness: guard quit during import, sweep leftover encode temp files, ignore non-video drops on Library
• Copy/polish: honest crop preset state, completion settings shortcut, clearer toolbar icon and placeholder, remove jargon and third-party references from user-facing copy
• Docs: cover polish checks in TESTING

Commits

$(git log --oneline origin/main..HEAD)

Verification

• swift build / swift test pass (six test classes)
• Manual import/lock-unlock verification per TESTING.md