refactor: routing rewrite on TanStack Solid Router (@miodec)

[Miodec]

Replaces #8451. Pages move to SolidJS and are rendered by their TanStack Solid Router routes; the router's own pending, error and not-found handling replaces the hand-rolled page state. Decisions: docs/adr/0001-routes-render-pages.md, 0002-out-in-page-fade.md, 0003-pages-show-before-background-data.md; terms in GLOSSARY.md.

Summary

 <RouterProvider>
-  <Pages>                         # outside the router, staticData.page picks the page
-    hand-rolled fade, loading page, load error signals
+  <RootLayout>                    # out-in page fade around <Outlet/>
+    loading page                  # while the loading bar or pending spinner shows
+    <Outlet/>
+      <Page>                      # mounts content once attached, Suspense net
+        <AsyncContent>            # own Suspense per section
+  defaultPendingComponent         # loading page + spinner after 500ms, kept ≥500ms
+  defaultErrorComponent           # loading page error + Retry (router.invalidate)
+  notFoundComponent               # 404 page, replaces the `$` route

Navigation:

link hover            -> intent preload (preload-safe: no loading steps, no global state)
onBeforeNavigate      -> leaving the test page stops the test before the fade out
history blocker       -> test restarting, result calculating, no-quit funbox, loading bar shown
app beforeLoad        -> first auth wait, plans one loading bar for the navigation
  loading steps       -> user data, results (account), friends; each a share of 0-90, then "Done"
loaders               -> background data: prefetchQuery, not awaited

• Routes read auth and queryClient from router context.
• Typed navigation everywhere (navigate({ to, params, search })); Button / Balloon are real links (createLink, cmd+click).
• The URL is the source of truth for leaderboards, settings and account settings selections (validateSearch + useSearch, replace writes); ?mode2=15 keeps its format.

Evidence

• Before: no tests at the router boundary; old Pages.spec mocked page selection.
  After: app-shell tests drive the real route tree in a real RouterProvider with memory history, fake auth and a test query client: guards and redirects, loading bar plans (refresh on account 0-50 / 50-90 / Done, refresh on test 0-90 / Done, test -> account 0-90 / Done), fades (never two pages, rapid navigation ends on the last page), pending spinner (fake timers), errors + Retry, 404, preload safety, search params, leaving the test page. Frontend: 63 files, 1416 tests passing; type check clean.
• A dev-transform spec runs the dev Solid/Vite transform over every .tsx file (catches component patterns tests and lint miss, e.g. function overloads).
• Browser verification: in progress.

Merge Danger

Door: two-way

Frontend only, no API or data changes. Revertable.

Blast Radius: navigation

Every page change, first load, sign in/out redirects, and leaving the test page go through new code. Known behaviour changes:

• Bare /leaderboards redirects to the last shown selection.
• Navigation is blocked while the loading bar is shown.
• zod-urlsearchparams dependency removed.
• Leaderboard entries are preloaded on hover for the hovered selection only.

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