test(e4): loadInitialPage precedence + channelViewFrom (S1 fix)

Split the two URL-derived nav seeds out of NavigationProvider into
navigationInit.ts so they're unit-testable and the provider stays
react-refresh-clean (component-only exports). channelViewFrom also dedups the
identical _chan/?chan resolve the channelView seed and the onPop handler each
hand-rolled — locking the S1 fix (a popstate on a URL-only bookmark entry must
restore the channel from ?chan, not close it). Pure code motion.

Tests: the full loadInitialPage precedence matrix (?page > ?v > ?pl >
filters/?chan > stored landing > feed, with the beats-a-lower-param cases) and
channelViewFrom (_chan+name, ?chan fallback, state-over-url, closed → null).

15 tests; full FE gate green (tsc/eslint/prettier/knip).

frontend/src/components/navigationInit.ts

@@ -0,0 +1,39 @@
+// URL-derived navigation seeds, split out of NavigationProvider so they stay unit-testable AND the
+// provider file keeps react-refresh happy (component-only exports). `loadInitialPage` picks the landing
+// module from a bookmark's params; `channelViewFrom` is the shared _chan/?chan resolver behind BOTH the
+// channelView seed and the onPop handler.
+import { hasFilterParams, isPage, readPage, type Page } from "../lib/urlState";
+import { getAccountRaw, LS } from "../lib/storage";
+
+const PAGE_KEY = LS.page;
+
+export interface ChannelView {
+  id: string;
+  name?: string;
+}
+
+export function loadInitialPage(): Page {
+  const params = new URLSearchParams(window.location.search);
+  if (params.has("page")) return readPage();
+  // A view-specific param implies its page, so a bookmarked deep link lands on the right module even
+  // without an explicit ?page. Feed filters (or a channel overlay) imply the feed.
+  if (params.has("v")) return "plex";
+  if (params.has("pl")) return "playlists";
+  if (hasFilterParams(params) || params.has("chan")) return "feed";
+  // A bare URL → the account's stored landing page.
+  const stored = getAccountRaw(PAGE_KEY);
+  return isPage(stored) ? stored : "feed";
+}
+
+/** Resolve the open channel page from a landed history entry: the in-app `_chan` id (with its optional
+ *  `_chanName`), else the ?chan URL param — a fresh ?chan bookmark's entry is URL-only. The seed AND
+ *  the popstate handler share this ONE resolver so a popstate that lands on a bookmark entry (no
+ *  `_chan`) restores the channel from the URL instead of wrongly closing it. */
+export function channelViewFrom(
+  state: { _chan?: unknown; _chanName?: unknown } | null | undefined,
+  search: string,
+): ChannelView | null {
+  const id =
+    (state?._chan as string | undefined) ?? new URLSearchParams(search).get("chan") ?? undefined;
+  return id ? { id, name: (state?._chanName as string) ?? undefined } : null;
+}