feat: replace textarea with TipTap rich text editor

- Add TipTap WYSIWYG editor (headless @tiptap/core) with StarterKit,
  TaskList, TaskItem, Link, Placeholder, and tiptap-markdown extensions
- Always-visible formatting toolbar with undo/redo, H1-H3, bold, italic,
  strikethrough, inline code, code block, blockquote, bullet/ordered/task
  lists, horizontal rule, and link
- Raw markdown toggle for power users
- Render rich text (HTML) on note cards instead of stripped plain text
- Update all e2e tests for TipTap contenteditable editor

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/e2e/search.spec.ts

@@ -23,7 +23,9 @@ test.describe('Search', () => {
 		// Given a note titled "Recipe" with content "Pasta with tomato sauce" exists
 		await page.getByTestId('new-note-btn').click();
 		await page.getByTestId('note-title-input').fill('Recipe');
-		await page.getByTestId('note-content-input').fill('Pasta with tomato sauce');
+		const editor = page.getByTestId('tiptap-editor').locator('.tiptap');
+		await editor.click();
+		await editor.pressSequentially('Pasta with tomato sauce');
 		await page.getByTestId('close-editor-btn').click();
 
 		// When the user searches for "tomato"