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/2026-02-20/move_to_library_keyboard_shortcuts.md

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+# Move to Library — Keyboard Shortcut Buttons
+
+**Date:** 2026-02-20
+
+## Overview
+
+Replace the dropdown-based library picker in the "Move to Library" dialog with a horizontal row of buttons. Each button represents a target library and has one letter highlighted (underlined) as a keyboard shortcut. Pressing that key immediately selects and executes the move.
+
+## UX Design
+
+- Libraries are displayed as **buttons in a horizontal, wrapping row** (flexbox wrap).
+- Each button shows the library name with **one letter visually highlighted** (e.g. underlined, bold, or different color) to indicate its keyboard shortcut.
+- Letters are assigned greedily: use the **first unused letter** of each library name, scanning left to right. This ensures the mnemonic is intuitive.
+- When the dialog is open, pressing a shortcut key triggers the move immediately (same as clicking the button).
+- If there are multiple folders in the target library, fall back to the existing folder dropdown before executing. 
+
+## Letter Assignment Algorithm
+
+```
+used = {}
+for each library in targetLibraries:
+  for each character in library.name:
+    letter = character.toLowerCase()
+    if letter is a-z and letter not in used:
+      assign letter as shortcut
+      used.add(letter)
+      break
+```
+
+## Keyboard Handling
+
+- Listen for `keydown` events while the dialog is open.
+- Match the pressed key against assigned shortcuts (case-insensitive).
+- If matched and not processing, trigger `moveItems()` for that library.
+- Ignore key events when a child input/select element is focused (e.g., folder dropdown).
+
+## Visual Button Design
+
+Each button should:
+- Show the library name with the shortcut letter underlined.
+- Have a hover/focus state.
+- Show an active/selected state when a library is picked.
+- The shortcut letter should be highlighted using a `<span>` with distinct styling (underline + slightly different color).