feat(mail): replace contentEditable with TipTap rich text editor

- Add RichTextEditor component with TipTap (ProseMirror-based) editor
- Full toolbar: bold, italic, underline, strikethrough, H1/H2/H3, bullet/numbered lists,
  blockquote, inline code, code block, text alignment (left/center/right/justify),
  text color picker, link insert/edit, image insert, clear formatting, undo/redo
- Replace old contentEditable div + document.execCommand in ComposeModal
- Add Placeholder extension for empty editor hint
- Add ProseMirror CSS styles (lists, blockquote, code, headings, links, images)
- Update signature/template insertion to work with new editor state

frontend/src/index.css

@@ -91,3 +91,66 @@
 ::-webkit-scrollbar-thumb:hover {
   background: #94a3b8;
 }
+
+/* TipTap editor placeholder */
+.ProseMirror p.is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: #94a3b8;
+  pointer-events: none;
+  height: 0;
+}
+
+/* TipTap editor base styles */
+.ProseMirror {
+  outline: none;
+}
+.ProseMirror ul,
+.ProseMirror ol {
+  padding-left: 1.5rem;
+}
+.ProseMirror ul {
+  list-style-type: disc;
+}
+.ProseMirror ol {
+  list-style-type: decimal;
+}
+.ProseMirror blockquote {
+  padding-left: 1rem;
+  border-left: 3px solid #e2e8f0;
+  margin-left: 0;
+  color: #64748b;
+  font-style: italic;
+}
+.ProseMirror pre {
+  background: #1e293b;
+  color: #e2e8f0;
+  border-radius: 0.375rem;
+  padding: 0.75rem 1rem;
+  font-family: monospace;
+  font-size: 0.875rem;
+  overflow-x: auto;
+}
+.ProseMirror code {
+  background: #f1f5f9;
+  border-radius: 0.25rem;
+  padding: 0.125rem 0.25rem;
+  font-size: 0.875em;
+}
+.ProseMirror pre code {
+  background: none;
+  padding: 0;
+}
+.ProseMirror img {
+  max-width: 100%;
+  height: auto;
+  border-radius: 0.375rem;
+}
+.ProseMirror a {
+  color: #2563eb;
+  text-decoration: underline;
+  cursor: pointer;
+}
+.ProseMirror h1 { font-size: 1.5em; font-weight: 700; margin: 0.5em 0; }
+.ProseMirror h2 { font-size: 1.25em; font-weight: 700; margin: 0.5em 0; }
+.ProseMirror h3 { font-size: 1.1em; font-weight: 600; margin: 0.5em 0; }