fix: dashboard project tree rendering and session selection

- Use data-project attribute instead of index for project identification
- Fix toggleProject to use nextElementSibling for content panel
- Highlight selected session without re-rendering tree
- Add selected-session CSS class

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

frontend/static/common.css

@@ -406,3 +406,8 @@ body {
 .w-full { width: 100%; }
 .truncate { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
 .hidden { display: none !important; }
+
+.selected-session {
+    background: var(--bg-tertiary) !important;
+    border-left: 3px solid var(--accent-color);
+}