feat: resolve Phase 1 contract requests and compose every feature

Contracts and core:
- Feature gains auth and stop; FeatureFactory; core composeServer builds the
  context, runs the factories, then the job queue from the union of their jobs
  (R8/R14). startServer uses it; main.ts composes learner, import, studio, AI.
- AppConfig/loadConfig own the layout and settings: courseDirs include
  <dataDir>/courses and <dataDir>/worktrees, studioReposDir, cacheDir,
  importRoots, uploadMaxMb, aiAllowedBaseUrls, transcribeUrl, python.
  resolveConfig derives the same for tests; cachePaths shares the importer's
  cache layout.
- canView: the global author role views every course (R14); the learner,
  studio and AI packages all call it.
- migrate(db, packages): core first, then packages in name order.
- Media: nosniff on every response, a sandbox CSP on svg/html/js; a Range on
  an empty file is 416.
- courseUrl/lessonUrl and the lesson route patterns live in @stc/contracts.

Packages:
- import, studio and AI read AppConfig instead of process.env; the importer
  refuses ids used by studio/repos.
- learner migrations run through core migrate; library covers fall back to
  the importer's cached candidate behind a canView-checked route.
- studio preview renders through the learner LessonView (anchors honoured,
  draft URLs); AI drafts go to the studio draft branch for studio courses (as
  the requesting user) and to <dataDir>/drafts otherwise, and the generator
  page says which.

Web: tsconfig paths for @stc/<feature>/web browser entries; features.ts
imports them by package name; nav.roles hides links by role.

Tests: compose, config, canView, media headers, migrations order, covers,
preview, nav, SPA bundle without server code, and an end-to-end flow on
SQLite and Postgres.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/main.tsx

@@ -1,15 +1,33 @@
 import { createRoot } from "react-dom/client";
-import { lazy, Suspense } from "react";
-import { createBrowserRouter, Link, Outlet, RouterProvider, type RouteObject } from "react-router-dom";
+import { lazy, Suspense, useEffect, useState } from "react";
+import { createBrowserRouter, Link, Outlet, RouterProvider, useLocation, type RouteObject } from "react-router-dom";
+import type { AuthUser } from "@stc/contracts";
 import { AppShell } from "@stc/ui";
 import { webFeatures } from "./features";
+import { visibleNav } from "./nav";
 
 function Home() {
   return <p className="work">No features installed yet</p>;
 }
 
+/** The signed-in user (GET /api/me), refreshed on navigation so sign-in and sign-out update the nav. */
+function useSessionUser(): AuthUser | null {
+  const { pathname } = useLocation();
+  const [user, setUser] = useState<AuthUser | null>(null);
+  useEffect(() => {
+    let live = true;
+    fetch("/api/me", { credentials: "same-origin" })
+      .then((r) => (r.ok ? r.json() : { user: null }))
+      .then((b: { user: AuthUser | null }) => { if (live) setUser(b.user ?? null); })
+      .catch(() => { if (live) setUser(null); });
+    return () => { live = false; };
+  }, [pathname]);
+  return user;
+}
+
 function Layout() {
-  const nav = webFeatures.flatMap((f) => f.routes.filter((r) => r.nav?.area === "header"));
+  // Links the user cannot use are hidden; the pages and APIs still enforce access themselves.
+  const nav = visibleNav(webFeatures, useSessionUser());
   const header = (
     <>
       <Link to="/" style={{ fontWeight: 600, color: "var(--head)", textDecoration: "none" }}>STCourses</Link>