feat(ui): add app shell, router, and base path handling

Base path comes from BUN_PUBLIC_BASE_PATH so the same bundle serves both
a root Docker deploy and a GitHub Pages subpath.

Deviation from the task-16 brief: does not install @happy-dom/global-registrator,
does not create test/setup.ts, and does not add a [test] preload to bunfig.toml.
A global DOM preload would make src/domain/smoke.test.ts's "importable without a
DOM" assertion vacuously true, silently retiring the check that guards the
domain-layer-stays-pure invariant. Nothing in this task needs a DOM: basePath.test.ts
is pure string logic, and the shell render test was intentionally omitted by the
brief itself. A later task will register happy-dom per-file where a DOM is
genuinely required.

src/ui/routes/MonthRoute.tsx

@@ -0,0 +1,6 @@
+import { useParams } from "react-router";
+
+export function MonthRoute() {
+  const { monthId } = useParams();
+  return <h1 className="text-2xl font-semibold">{monthId}</h1>;
+}

src/ui/routes/PostMonthRoute.tsx

@@ -0,0 +1,10 @@
+import { useParams } from "react-router";
+
+export function PostMonthRoute() {
+  const { postId, monthId } = useParams();
+  return (
+    <h1 className="text-2xl font-semibold">
+      Post {postId} — {monthId}
+    </h1>
+  );
+}

src/ui/routes/PostYearRoute.tsx

@@ -0,0 +1,10 @@
+import { useParams } from "react-router";
+
+export function PostYearRoute() {
+  const { postId, year } = useParams();
+  return (
+    <h1 className="text-2xl font-semibold">
+      Post {postId} — Year {year}
+    </h1>
+  );
+}

src/ui/routes/SettingsRoute.tsx

@@ -0,0 +1,3 @@
+export function SettingsRoute() {
+  return <h1 className="text-2xl font-semibold">Settings</h1>;
+}

src/ui/routes/SummaryRoute.tsx

@@ -0,0 +1,3 @@
+export function SummaryRoute() {
+  return <h1 className="text-2xl font-semibold">Summary</h1>;
+}

src/ui/routes/YearRoute.tsx

@@ -0,0 +1,6 @@
+import { useParams } from "react-router";
+
+export function YearRoute() {
+  const { year } = useParams();
+  return <h1 className="text-2xl font-semibold">Year {year}</h1>;
+}