Bottom-anchor banner artwork, instant navigation via in-memory page cache, seed/stars/structure tweaks

- Fix banner artwork sizing: explicit 100% grid track so children
  stretch instead of resolving height:100% against a content-sized row
  (SVG intrinsic ratio bloated the row, cropping the artwork's bottom).
  Bottom-anchor via object-position, transform-origin and YMax slice.
- pagerite.js: in-memory page cache — preload every visible internal
  link once, serve navigation from memory without fetching; editors'
  loadPlain keeps the cache in sync (pagerite:page-fetched).
- Structure editor: delete pages directly, no two-step confirmation.
- New 'stars' banner design (drifting starfield) alongside 'eyes'.
- Rewrite seed content: welcome page, three-level docs section covering
  all Markdown features (source + rendered), showcase hierarchy with
  image positioning and a simple leaf-page banner example.

frontend/src/StructureTree.vue

@@ -144,12 +144,11 @@ function onEnd() {
                 v-if="element.has_content || !element.children.length"
                 type="button"
                 class="act del"
-                :class="{ armed: handlers.arming() === element.path }"
                 :title="element.children.length
                   ? 'delete the landing page (the category keeps its subpages)'
                   : 'delete page'"
-                @click="handlers.armRemove(element)"
-              >{{ handlers.arming() === element.path ? 'delete?' : '✕' }}</button>
+                @click="handlers.removePage(element)"
+              >✕</button>
             </span>
           </template>
         </div>
@@ -300,12 +299,6 @@ body.tree-dragging .treelist {
   white-space: nowrap;
 }
 
-/* Two-step delete: the first click arms the button, the second deletes. */
-.act.armed {
-  color: #e06c75;
-  font-weight: 600;
-}
-
 .del:hover {
   color: #e06c75;
 }