refactor(ui): extract bindThumbnailFallbacks from app.js

Co-authored-by: PR0M3TH3AN <[email protected]>

docs/agents/task-logs/weekly/2026-02-23_01-15-37_refactor-agent_completed.md

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+# Refactor Agent: Extract bindThumbnailFallbacks
+
+## Summary
+Extracted the `bindThumbnailFallbacks` logic from `js/app.js` into a new dedicated UI utility module `js/ui/thumbnailBinder.js`. This reduces the complexity of the monolithic `Application` class and improves testability of the thumbnail fallback logic.
+
+## Changes
+- Created `js/ui/thumbnailBinder.js`.
+- Extracted `bindThumbnailFallbacks` function and `FALLBACK_THUMBNAIL_SRC` constant.
+- Updated `js/app.js` to import and delegate to the new module.
+- Added unit tests in `tests/unit/ui/thumbnailBinder.test.mjs`.
+
+## Verification
+- `npm run test:unit`: Passed (including new tests).
+- `npm run lint`: Passed.
+
+## AGENTS.md Refactor Checklist Compliance
+- **Select one refactor target**: Selected `bindThumbnailFallbacks` (>50 LOC, mix of concerns).
+- **Extract one coherent helper**: Extracted `bindThumbnailFallbacks`.
+- **Wire thin wrapper**: `Application.prototype.bindThumbnailFallbacks` now delegates to the imported function.
+- **Add unit tests**: Added comprehensive tests covering normal flow and edge cases.
+- **Produce small PR**: Changes are limited to extraction and wiring.
+- **Preserve semantics**: Original behavior, including event binding logic and fallback source selection, is preserved.