chore(frontend): add Tiptap placeholder CSS and lock Tiptap deps

Placeholder uses ::before pseudo-element on the contenteditable's
data-placeholder attribute, only visible when the editor is unfocused
and empty. Removes the default ProseMirror focus ring since the outer
wrapper provides its own.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/layout.css

@@ -416,6 +416,23 @@
 	}
 }
 
+/* ─── Tiptap editor ─────────────────────────────────────────────────────────── */
+
+/* Placeholder: shown on the inner contenteditable when the editor is empty
+   and unfocused. Uses the data-placeholder attribute set via editorProps. */
+.tiptap-editor-inner[data-placeholder]:not(:focus)::before {
+	content: attr(data-placeholder);
+	color: var(--c-ink-3);
+	pointer-events: none;
+	float: left;
+	height: 0;
+}
+
+/* Remove default ProseMirror focus ring — the outer wrapper has its own. */
+.tiptap-editor-inner:focus {
+	outline: none;
+}
+
 @keyframes slide {
 	0% {
 		transform: translateX(-100%);