Free drag & drop with fixed positions, empty cells and kept pages (0.2.31)

Tiles get an absolute grid slot instead of a packed order: a tile lands exactly
where it is dropped, dropping on another tile swaps the two, everything else
stays put, and empty cells and empty pages persist. The mockup previews the
result while dragging, a new page waits below the last one, "Pagina toevoegen"
keeps an empty page, empty cells accept the next tile from the picker, and the
mouse no longer selects text while dragging.

Storage and protocol stay version 1: `tiles[].slot` (0-47) and `pages` (1-8)
are additive, the layout message carries `slots` and `pages`. Firmware 0.2.26
places tiles on those slots (`place()` next to `pack()`); older firmware packs
in order as before and the editor says so.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

screen_manager/app/static/index.html

@@ -98,9 +98,13 @@
                 <span id="count">0 / 10</span>
               </div>
               <div id="layout-preview" aria-label="Schermindeling"></div>
+              <button id="add-page" class="quiet add-page" type="button">
+                ＋ Pagina toevoegen
+              </button>
               <p id="no-tiles" class="hint">
                 Voeg je eerste lamp, scène of apparaat toe via de kiezer.
               </p>
+              <p id="positions-hint" class="hint" hidden></p>
             </div>
             <div class="picker">
               <label for="search">Voeg een entiteit toe</label