UI improvements: fix keyboard shortcuts, search highlighting, strength meter on register, colored folders, shortcut help

- Rewrite keyboard shortcuts using e.code and early preventDefault() to reliably override browser defaults
- Add ? key and toolbar button for shortcuts help modal
- Add password strength meter to register form
- Add search highlighting in all view modes (grid/list/compact/table)
- Add hash-based color coding for folder chips
- Add highlightText utility with regex escaping

index.html

@@ -77,9 +77,10 @@
             <div id="registerForm" class="hidden">
                 <div class="input-group">
                     <input type="text" id="regUsername" placeholder="Username (min 3)" autocomplete="off">
-                    <input type="password" id="regPassword" placeholder="Password (min 8)" autocomplete="off">
+                    <input type="password" id="regPassword" placeholder="Password (min 8)" autocomplete="off" oninput="checkRegStrength()">
                     <button class="btn" id="registerBtn" onclick="register()">✨ Create</button>
                 </div>
+                <div class="strength-bar s0" id="regStrengthBar" style="margin-top:0.4rem;margin-bottom:0.2rem;flex-basis:100%"></div>
             </div>
         </div>
 
@@ -149,6 +150,7 @@
                 <div style="display:flex;gap:.4rem;align-items:center;flex-wrap:wrap">
                     <button class="btn btn-outline btn-sm" id="trashBtn" onclick="toggleTrash()" title="Trash">🗑️</button>
 					<button class="btn btn-outline btn-sm" id="selectBtn" onclick="toggleSelectMode()" title="Batch select">☐ Select</button>
+					<button class="btn btn-outline btn-sm" onclick="showShortcutsHelp()" title="Shortcuts">⌨️</button>
                     <div class="view-toggle" id="viewToggle">
                         <button class="view-btn active" data-view="grid">🟫 Grid</button>
                         <button class="view-btn" data-view="compact">📝 Compact</button>