Fix hero-canvas navigation race; style the prosekit editor and add a toolbar

- Hero: gate RasterizedYES construction on a NodeRef resolving, not
  just "the Effect ran". Root cause of the reported "loading..." stall
  + "RefCell already borrowed" panic - navigating back to / client-side
  could run the Effect before the new <canvas> was actually in the
  DOM, yes.js did an unchecked getElementById(...).getContext() on
  null and threw mid-reactive-update, corrupting wasm_bindgen_futures'
  executor badly enough to panic on the next tick. Same NodeRef-gating
  pattern the prosekit editor's own mount already used.
- style/main.css: .prosekit-wrap/.prosekit-toolbar/.prosekit-editor to
  match the existing input/textarea look (border, background, focus
  ring).
- prosekit-editor.js: a real toolbar (Bold, Italic, H1, H2, Link, and
  a dedicated Gitea-repo-embed button reusing the paste rule's own
  insert logic) with active-state highlighting via the standard
  ProseMirror markActive/nodeActive idiom.

style/main.css

@@ -218,6 +218,59 @@ input:focus, textarea:focus {
   box-shadow: 0 0 0 3px var(--accent-soft);
 }
 
+.prosekit-wrap {
+  background: var(--paper);
+  border: 1px solid var(--line);
+  border-radius: 0.6rem;
+  overflow: hidden;
+}
+
+.prosekit-toolbar {
+  display: flex;
+  gap: 0.2rem;
+  padding: 0.4rem;
+  border-bottom: 1px solid var(--line);
+  flex-wrap: wrap;
+}
+
+.prosekit-toolbar button {
+  font: inherit;
+  font-size: 0.9rem;
+  color: var(--ink-dim);
+  background: transparent;
+  border: 1px solid transparent;
+  border-radius: 0.4rem;
+  padding: 0.3rem 0.55rem;
+  cursor: pointer;
+  line-height: 1;
+}
+
+.prosekit-toolbar button:hover { color: var(--ink); border-color: var(--line); }
+
+.prosekit-toolbar button.is-active {
+  color: var(--paper);
+  background: var(--accent);
+}
+
+.prosekit-editor {
+  font: inherit;
+  color: var(--ink);
+  padding: 0.65rem 0.8rem;
+  min-height: 5rem;
+}
+
+.prosekit-editor:focus {
+  outline: none;
+}
+
+.prosekit-wrap:focus-within {
+  border-color: var(--accent);
+  box-shadow: 0 0 0 3px var(--accent-soft);
+}
+
+.prosekit-editor p { margin: 0 0 0.6em; }
+.prosekit-editor p:last-child { margin-bottom: 0; }
+
 .alt-submit {
   margin-top: 1.4rem;
   font-family: var(--sans);