gitea: default repo setting; editor: asset picker with thumbnail grid

Gitea plugin: default_repo field per account (stored in DB, pre-fills
create issue form but stays editable). Editor toolbar: asset picker
flyout with 4-column thumbnail grid and live search filter. Clicking
a tile inserts the matching markdown.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/index.php

@@ -2203,6 +2203,7 @@ if (str_starts_with($route, '/api/')) {
                 <button data-md="extract" title="Extract selection to new card" class="toolbar-btn"><i class="ph ph-scissors text-sm"></i></button>
                 <button id="md-upload-pdf" title="Upload & link PDF" class="toolbar-btn"><i class="ph ph-file-pdf text-sm"></i></button>
                 <input type="file" id="md-upload-pdf-file" accept=".pdf,application/pdf" class="hidden">
+                <button id="md-asset-picker" title="Insert from assets" class="toolbar-btn" style="position:relative;"><i class="ph ph-images text-sm"></i></button>
                 <div class="w-px h-4 bg-t-border mx-1"></div>
                 <button id="md-find-replace" title="Find & replace (Ctrl+H)" class="toolbar-btn" onclick="openFindBar(true)"><i class="ph ph-magnifying-glass text-sm"></i></button>
                 <?= PluginManager::instance()->slot('md-toolbar') ?>