feat(web): add client-side routes for views

Deep-link, refresh and back/forward now land on the view the operator
was on instead of the board. Vanilla History API (pushState/popstate),
no router dependency: /library, /queues, /movies/{id}/releases and
/search?q= all restore on load.

Closes #102

web/src/router.ts

@@ -0,0 +1,70 @@
+// Vanilla History API routing, no router dependency (see the routes issue). The URL
+// is the source of truth: every route is derived from `location`, never from
+// state passed to pushState, so a manually edited or copy-pasted URL works.
+
+export type Route =
+  | { kind: "board" }
+  | { kind: "library" }
+  | { kind: "queues" }
+  | { kind: "search"; query: string }
+  | { kind: "releases"; movieId: number };
+
+export function parseRoute(url: URL): Route {
+  const segments = url.pathname.split("/").filter(Boolean);
+  if (segments.length === 1 && segments[0] === "library") {
+    return { kind: "library" };
+  }
+  if (segments.length === 1 && segments[0] === "queues") {
+    return { kind: "queues" };
+  }
+  if (segments.length === 1 && segments[0] === "search") {
+    const query = url.searchParams.get("q") ?? "";
+    return query === "" ? { kind: "board" } : { kind: "search", query };
+  }
+  if (segments.length === 3 && segments[0] === "movies" && segments[2] === "releases") {
+    const movieId = Number(segments[1]);
+    if (Number.isInteger(movieId) && movieId > 0) {
+      return { kind: "releases", movieId };
+    }
+  }
+  return { kind: "board" };
+}
+
+export function routePath(route: Route): string {
+  switch (route.kind) {
+    case "board":
+      return "/";
+    case "library":
+      return "/library";
+    case "queues":
+      return "/queues";
+    case "search":
+      return `/search?q=${encodeURIComponent(route.query)}`;
+    case "releases":
+      return `/movies/${route.movieId}/releases`;
+  }
+}
+
+export function currentRoute(): Route {
+  return parseRoute(new URL(location.href));
+}
+
+/**
+ * Updates the URL to `route`. `replace: true` rewrites the current entry
+ * instead of pushing one — used for search-as-you-type, so every keystroke
+ * doesn't add a back-stack entry.
+ */
+export function navigate(route: Route, options: { replace?: boolean } = {}) {
+  const path = routePath(route);
+  if (location.pathname + location.search === path) {
+    if (options.replace) {
+      history.replaceState(null, "", path);
+    }
+    return;
+  }
+  if (options.replace) {
+    history.replaceState(null, "", path);
+  } else {
+    history.pushState(null, "", path);
+  }
+}