the preview never had a tab: the URL param is :section, not :tabId

Selecting a tab put its id in the URL and the preview still said "Select a tab to preview".

A plugin gets exactly two routes from the shell — /<route> and /<route>/:section — so :section is the
only param name that ever arrives, whatever the plugin means by it. This read :tabId, which was right
while it was a platform screen with its own /browser/:tabId route and stayed in place through the
extraction. The route matched, the id was in the URL, and useParams().tabId was undefined.

Nothing typechecks this: the param is a string key, and the wrong one is simply absent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/shared.ts

@@ -17,7 +17,16 @@ export type BrowserTarget = {
 export const browserTabPath = (id: string) => `/browser/${encodeURIComponent(id)}`;
 
 /** The previewed tab, or null on the bare `/browser` route — the list with nothing open. */
-export const useSelectedTabId = (): string | null => useParams<{ tabId?: string }>().tabId ?? null;
+/**
+ * Which tab is open, from the URL.
+ *
+ * The param is `section`, not `tabId`. A plugin gets exactly two routes from the shell — `/<route>` and
+ * `/<route>/:section` — so `:section` is the only param name that ever arrives, whatever the plugin
+ * means by it. This read `tabId` while it was a platform screen with its own `/browser/:tabId` route,
+ * and kept reading it after becoming a plugin: the id was in the URL, the route matched, and the panel
+ * saw undefined. The preview said "Select a tab to preview" with a tab plainly selected.
+ */
+export const useSelectedTabId = (): string | null => useParams<{ section?: string }>().section ?? null;
 
 // Both panels want this list and React Query serves both observers from one key, so the preview being
 // able to tell "no tab open" from "that tab is gone" costs no extra request. It needs to: a CDP target