feat(frontend): Scholarchat-inspired UI refactor with top nav, sidebar, and message actions

- Add TopNav component with AI Chat, Study Guides, Progress tabs
- Refactor sidebar to include brand, new chat, subjects, chat history, settings
- Add message action buttons (Helpful, Note, Deeper) on assistant responses
- Enhance ChatInput with photo, mic attachment icons and AI disclaimer
- Improve welcome state with 2x2 suggested prompt grid
- Update global layout for proper sidebar offset (fix nav overlap)
- Remove obsolete Header component
- All styles: strict BEM naming, zero inline CSS, Paper Look theme

frontend/src/components/message/Message.jsx

@@ -0,0 +1,45 @@
+/* ========================================
+   Message Component — padhLe
+   ======================================== */
+
+import "./Message.css";
+
+const Message = ({ message }) => {
+  const { role, text } = message;
+  const isUser = role === "user";
+
+  return (
+    <div className={`message ${isUser ? "message--user" : "message--assistant"}`}>
+      <div className="message__bubble">
+        <div className="message__text">{text}</div>
+      </div>
+
+      {/* Action buttons only for assistant messages */}
+      {!isUser && (
+        <div className="message__actions">
+          <button className="message__action" aria-label="Helpful" title="Helpful">
+            <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+              <path d="M7 22h4c1.1 0 2-.9 2-2v-4c0-.57-.24-1.09-.62-1.47L13 11.76c.38-.38.62-.9.62-1.47V7c0-1.1-.9-2-2-2H7c-1.1 0-2 .9-2 2v12c0 3.31 2.69 6 6 6h8v-2h-2.11c-.18-.5-.29-1.04-.29-1.6V22z" />
+            </svg>
+            <span>Helpful</span>
+          </button>
+          <button className="message__action" aria-label="Save note" title="Save Note">
+            <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+              <path d="M19 21l-7-5-7 5V5a2 2 0 0 1 2-2h10a2 2 0 0 1 2 2z" />
+            </svg>
+            <span>Note</span>
+          </button>
+          <button className="message__action" aria-label="Explain deeper" title="Explain Deeper">
+            <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+              <path d="M12 2a10 10 0 1 0 10 10H12V2z" />
+              <path d="M12 2a10 10 0 0 1 10 10" />
+            </svg>
+            <span>Deeper</span>
+          </button>
+        </div>
+      )}
+    </div>
+  );
+};
+
+export default Message;