feat(editor): add TitleEditor component, replace <input> for issue titles

- New TitleEditor: minimal tiptap (Document+Paragraph+Text+Placeholder)
- Single-paragraph constraint prevents Enter from creating new lines
- contenteditable div enables visual word-wrap (no horizontal scroll)
- Enter→submit+blur, Shift+Enter blocked, Escape→blur
- Replace <Input> in create-issue modal and <input> in issue-detail
- Remove titleDraft state/titleFocusedRef/sync effect from issue-detail
- Fix duplicate React key: TitleEditor key={`title-${id}`}

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

apps/web/components/common/title-editor.css

@@ -0,0 +1,18 @@
+/* Title editor: minimal ProseMirror for single-line titles */
+
+.title-editor.ProseMirror {
+  outline: none;
+}
+
+.title-editor.ProseMirror p {
+  margin: 0;
+}
+
+/* Placeholder */
+.title-editor .is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: var(--muted-foreground);
+  pointer-events: none;
+  height: 0;
+}