feat: add bookmarks with toolbar button, sidebar list, and keyboard shortcut

- bookmark button 🔖 in toolbar (Ctrl+B / Cmd+B shortcut)
- addBookmark() saves current position (CFI for epub, page for pdf, scroll% for text)
- bookmarks stored in localStorage per-file (bookish_bookmarks_<filename>)
- sidebar Segnalibri tab shows clickable list with delete button
- gotoBookmark() restores the exact position per format type
- CSS styles for bm-entry, bm-label, bm-time, bm-delete

bookish/templates/reader.html

@@ -26,6 +26,7 @@
             <button class="tool-btn" id="zoomIn" title="Zoom +">A+</button>
             <button class="tool-btn" id="themeToggle" title="Tema">☀️</button>
             <button class="tool-btn" id="searchToggle" title="Cerca">🔍</button>
+            <button class="tool-btn" id="bookmarkToggle" title="Segnalibro (Ctrl+B)">🔖</button>
             <button class="tool-btn" id="fullscreenToggle" title="Schermo intero">⛶</button>
         </div>
     </div>