refactor(tiles): the picker's position comes from its stylesheet alone

openTilePicker set `position: fixed` inline, which .tile-picker-menu
already declares; the inline copy (carried over from the split picker,
whose menu has the same rule) is gone. The top/right offsets under the
Tiles button stay inline, since they are measured.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/tile-grid.js

@@ -452,8 +452,8 @@ Object.assign(CodemanApp.prototype, {
     document.body.appendChild(menu);
     const btn = document.querySelector('.btn-tile-grid');
     if (btn?.getBoundingClientRect) {
+      // Under the Tiles button (.tile-picker-menu is position: fixed).
       const rect = btn.getBoundingClientRect();
-      menu.style.position = 'fixed';
       menu.style.top = `${rect.bottom + 4}px`;
       menu.style.right = `${window.innerWidth - rect.right}px`;
     }