feat: sprint 2 - empty states, onboarding tour, dev tab optimization, expanded usernames

- Add renderEmpty() helper with icons and hints for peers/history/logs/rooms
- Implement onboarding tour (4 steps) with overlay, dots, skip/next
- Dev tab logs only poll when tab is visible (isDevTabVisible flag)
- Expand username generation from 12/12 to 30/30 adjective-noun pairs
- Update ROADMAP.md to remove implemented features

extension/popup.html

@@ -425,5 +425,19 @@
     </div>
 
     <script src="popup.js" type="module"></script>
+
+    <!-- Onboarding Overlay -->
+    <div id="onboarding-overlay" style="display:none; position:fixed; inset:0; background:rgba(0,0,0,0.7); z-index:1000; align-items:center; justify-content:center;">
+        <div id="onboarding-card" style="background:var(--card); padding:24px; border-radius:16px; max-width:280px; width:90%; text-align:center; box-shadow:0 8px 32px rgba(0,0,0,0.5);">
+            <div id="onboarding-icon" style="font-size:48px; margin-bottom:12px;">\u{1F44B}</div>
+            <h2 id="onboarding-title" style="color:var(--accent); margin:0 0 8px; font-size:16px;">Welcome to KoalaSync!</h2>
+            <p id="onboarding-text" style="color:var(--text-muted); font-size:13px; margin:0 0 16px; line-height:1.4;">Let's get you started.</p>
+            <div style="display:flex; gap:8px; justify-content:center;">
+                <button id="onboarding-skip" class="secondary" style="width:auto; padding:8px 16px;">Skip</button>
+                <button id="onboarding-next" class="primary" style="width:auto; padding:8px 16px;">Next</button>
+            </div>
+            <div id="onboarding-dots" style="margin-top:12px; display:flex; gap:6px; justify-content:center;"></div>
+        </div>
+    </div>
 </body>
 </html>