Fix sidebar sections, terminal resize, and improve .gitignore

Sidebar:
- Add -webkit-user-select: none to section headers (WebKit prefix)
- Fix class mismatches: sidebar-section-chevron → section-chevron,
  sidebar-section-count → section-count, sidebar-section-list → sidebar-list

Terminal resize:
- Poll container dimensions after creation — retry fit every 50ms
  until container has non-zero width/height (up to 20 attempts)
- Add flex: 1 and min-height: 0 to .xterm inside session-panel
  so it participates in flex layout correctly

.gitignore:
- Comprehensive rewrite covering build output, Wails bindings,
  frontend libs, IDE files, OS metadata, and application data

frontend/src/css/sidebar.css

@@ -77,6 +77,7 @@
     letter-spacing: 0.06em;
     cursor: pointer;
     transition: color var(--transition-fast), background var(--transition-fast);
+    -webkit-user-select: none;
     user-select: none;
     text-align: left;
     background: none;

frontend/src/css/terminal.css

@@ -69,7 +69,9 @@
 /* xterm.js overrides for consistent look */
 .terminal-container .xterm,
 .session-panel .xterm {
+    flex: 1;
     height: 100%;
+    min-height: 0;
     padding: var(--space-1);
 }