feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

apps/web/src/styles.css

@@ -2321,3 +2321,138 @@ a.nav-item,
   gap: var(--space-2);
   align-items: center;
 }
+
+/* ============================================
+   Terminal Styles
+   ============================================ */
+
+.terminal-page {
+  display: flex;
+  flex-direction: column;
+  height: 100vh;
+  padding: var(--space-4);
+  gap: var(--space-4);
+}
+
+.terminal-page-header {
+  display: flex;
+  align-items: center;
+  gap: var(--space-4);
+  flex-shrink: 0;
+}
+
+.terminal-page-header h1 {
+  margin: 0;
+}
+
+.terminal-wrapper {
+  display: flex;
+  flex-direction: column;
+  flex: 1;
+  min-height: 0;
+  border: 1px solid var(--border);
+  border-radius: 10px;
+  overflow: hidden;
+  background: #1e1e1e;
+}
+
+.terminal-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: var(--space-3) var(--space-4);
+  background: #2d2d2d;
+  border-bottom: 1px solid #3e3e3e;
+  flex-shrink: 0;
+}
+
+.terminal-status {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+}
+
+.status-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  background: #666;
+}
+
+.status-dot.connecting {
+  background: #f5f543;
+  animation: pulse 1.5s infinite;
+}
+
+.status-dot.connected {
+  background: #0dbc79;
+}
+
+.status-dot.disconnected,
+.status-dot.error {
+  background: #cd3131;
+}
+
+@keyframes pulse {
+  0%,
+  100% {
+    opacity: 1;
+  }
+  50% {
+    opacity: 0.5;
+  }
+}
+
+.status-text {
+  font-size: 0.875rem;
+  color: #d4d4d4;
+  text-transform: capitalize;
+}
+
+.terminal-close {
+  padding: var(--space-1) var(--space-3);
+  background: transparent;
+  border: 1px solid #666;
+  border-radius: 6px;
+  color: #d4d4d4;
+  cursor: pointer;
+  font-size: 0.875rem;
+}
+
+.terminal-close:hover {
+  background: #3e3e3e;
+}
+
+.terminal-error {
+  padding: var(--space-3) var(--space-4);
+  background: #cd3131;
+  color: white;
+  font-size: 0.875rem;
+  flex-shrink: 0;
+}
+
+.terminal-container {
+  flex: 1;
+  min-height: 0;
+  padding: var(--space-2);
+}
+
+.terminal-container .xterm {
+  height: 100%;
+}
+
+.terminal-container .xterm-viewport {
+  background: #1e1e1e !important;
+}
+
+/* Responsive terminal */
+@media (max-width: 767px) {
+  .terminal-page {
+    padding: var(--space-2);
+    gap: var(--space-2);
+  }
+
+  .terminal-page-header h1 {
+    font-size: 1.25rem;
+  }
+}