refactor: rename frontend pages to PascalCase with Page suffix

Renamed 18 page files:
- dashboard.tsx → DashboardPage.tsx
- projects.tsx → ProjectsPage.tsx
- sessions.tsx → SessionsPage.tsx
- settings.tsx → SettingsPage.tsx
- ssh-keys.tsx → SshKeysPage.tsx
- terminal.tsx → TerminalPage.tsx
- tool-workshop.tsx → ToolWorkshopPage.tsx
- config-profiles.tsx → ConfigProfilesPage.tsx
- git-repositories.tsx → GitRepositoriesPage.tsx
- repo-workspace.tsx → RepoWorkspacePage.tsx
- workspaces.tsx → WorkspacesPage.tsx
- workspace-detail.tsx → WorkspaceDetailPage.tsx
- profile.tsx → ProfilePage.tsx
- project-settings.tsx → ProjectSettingsPage.tsx
- git-history.tsx → GitHistoryPage.tsx
- placeholder.tsx → PlaceholderPage.tsx

Updated router.tsx imports.

Quality gates: verified no remaining old imports.

apps/web/src/pages/PlaceholderPage.tsx

@@ -0,0 +1,37 @@
+export const PlaceholderPage = ({ title }: { title: string }) => {
+  return (
+    <section className="stack">
+      <h1>{title}</h1>
+      <p className="muted">This page is part of the frontend foundation scaffold.</p>
+    </section>
+  );
+};
+
+export const NotFoundPage = () => {
+  return (
+    <section className="stack center-screen">
+      <h1>404</h1>
+      <p className="muted">The page you requested does not exist.</p>
+    </section>
+  );
+};
+
+const API_BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
+
+import { Icon } from "../components/icon";
+
+export const LoginRedirectPage = () => {
+  const nextPath = new URLSearchParams(window.location.search).get("next") ?? "/";
+  const encodedNext = encodeURIComponent(nextPath);
+
+  return (
+    <section className="stack center-screen">
+      <h1>Sign in required</h1>
+      <p className="muted">You need to authenticate to access this section.</p>
+      <a className="primary-button" href={`${API_BASE_URL}/auth/login?next=${encodedNext}`}>
+        <Icon name="profile" size="sm" />
+        Continue to login
+      </a>
+    </section>
+  );
+};