feat: Move to Library dialog with keyboard shortcut buttons + Alt+M shortcut

- Replace library dropdown in MoveToLibraryModal with flex-wrap shortcut buttons
- Each button has one letter underlined as a keyboard shortcut (greedy first-unused-letter assignment)
- Pressing the shortcut key selects the library and immediately triggers the move
- If the target library has multiple folders, folder picker appears before moving
- Add Alt+M as an alias for the existing Ctrl+Shift+M Move to Library shortcut
- Update docs and artifacts index

artifacts/index.md

@@ -22,6 +22,7 @@ This index provides a quick reference for specification and documentation files
 | **2026-02-17** | [consolidate_singles.md](2026-02-17/consolidate_singles.md) | Expansion of the "Consolidate" feature to support single-file books by moving them into new folders. |
 | **2026-02-17** | [ui_enhancements.md](2026-02-17/ui_enhancements.md) | Specification for "View All" shortcuts on Home view shelves with specific sorting. |
 | **2026-02-20** | [promote_file_to_book.md](2026-02-20/promote_file_to_book.md) | Specification for "promoting" files from an existing book into a standalone library item, including a "Split Book" wizard. |
+| **2026-02-20** | [move_to_library_keyboard_shortcuts.md](2026-02-20/move_to_library_keyboard_shortcuts.md) | Specification for keyboard-shortcut-enabled library buttons in the "Move to Library" dialog. |
 | **General** | [docs/consolidate_feature.md](docs/consolidate_feature.md) | Comprehensive documentation for the "Consolidate" feature, including conflict resolution and technical details. |
 | **General** | [docs/item_restructuring_guide.md](docs/item_restructuring_guide.md) | Guide for Moving, Merging, and Splitting (Promoting) library items. |
 | **General** | [docs/metadata_management_tools.md](docs/metadata_management_tools.md) | Documentation for Reset Metadata and Batch Reset operations. |