feat: add Sessions Hub page

- Add Sessions tab to navigation between Dashboard and Projects
- Show active session count badge in navigation
- Create SessionsPage with:
  - Last session section with resume button
  - Active sessions grid with open/stop actions
  - Recent sessions list
  - Create session form with project/repo/tool selectors
- Add last_session_id to user config
- Update UserConfig schemas (backend and frontend)
- Add comprehensive CSS for sessions page

Quality gates: typecheck ✓, lint ✓, build ✓

apps/web/src/components/app-shell.tsx

@@ -11,6 +11,7 @@ import type { IconName } from "../utils/icons";
 
 const NAV_ITEMS: { to: string; label: string; icon: IconName }[] = [
   { to: "/", label: "Dashboard", icon: "dashboard" },
+  { to: "/sessions", label: "Sessions", icon: "terminal" },
   { to: "/projects", label: "Projects", icon: "projects" },
   { to: "/ssh-keys", label: "SSH Keys", icon: "profile" },
   { to: "/tool-types", label: "Tool Types", icon: "code" },
@@ -83,17 +84,24 @@ export const AppShell = () => {
 
       <div className="shell-body">
         <aside className="shell-nav" aria-label="Primary navigation">
-          {NAV_ITEMS.map((item) => (
-            <NavLink
-              key={item.to}
-              to={item.to}
-              className={({ isActive }) => (isActive ? "nav-item nav-item-active" : "nav-item")}
-              end={item.to === "/"}
-            >
-              <Icon name={item.icon} size="sm" />
-              {item.label}
-            </NavLink>
-          ))}
+          {NAV_ITEMS.map((item) => {
+            const isSessions = item.to === "/sessions";
+            const activeCount = sessions.filter((s) => s.status === "running").length;
+            return (
+              <NavLink
+                key={item.to}
+                to={item.to}
+                className={({ isActive }) => (isActive ? "nav-item nav-item-active" : "nav-item")}
+                end={item.to === "/"}
+              >
+                <Icon name={item.icon} size="sm" />
+                {item.label}
+                {isSessions && activeCount > 0 && (
+                  <span className="nav-badge">{activeCount}</span>
+                )}
+              </NavLink>
+            );
+          })}
           
           {sessions.length > 0 && (
             <>