PM-5935: Render a not-found page instead of a blank page for unowned paths

[jmgasper]

What was broken

The Topcoder opportunities page showed nothing at all. On
https://topcoder-dev.com/opportunities the Platform UI shell loaded (header, footer,
application container) but the content area stayed empty, and the browser console
reported No routes matched location "/opportunities".

Root cause

The website CloudFront viewer-request function now hands /opportunities and
/opportunities/* on the apex host to the Platform UI origin (topcoder-website commit
"Route opportunities to platform UI"), and the new Opportunities application is still on
the unmerged opportunities-v6 branch (PR #2147). Platform UI therefore serves the
request but has no route registered for that path.

PlatformRouter renders one <Route> per entry in platformRoutes, none of which
matched, so react-router rendered nothing and the user saw a blank page. Platform UI had
no fallback route at all, so the same blank page appeared for any path routed to it that
it does not own.

What was changed

• Added NotFoundPage under src/apps/platform/src/routes/not-found, reusing the
  existing ContentLayout, PageTitle, IconOutline and LinkButton components and
  mirroring the styling of the existing MemberNotFound page.
• Registered it in platformRoutes as a route: '*' catch-all, placed last in the list.

React-router ranks the * path lowest, so the fallback cannot shadow any route declared
above it, and routeGetActive never selects it because isActiveTool matches with
startsWith(route.route). On an app subdomain routeMatchAppRouter still replaces
allRoutes with that app's router, so subdomain behaviour is unchanged.

/opportunities now renders a clear "We were unable to find that page" message with a
link home instead of a blank page. Once the Opportunities application merges,
/opportunities matches its own route and the fallback is no longer reached.

Verified against a local yarn build:dev bundle

path	before	after
/opportunities	blank	not-found page
/opportunities/detail	blank	not-found page
/	home route	home route (unchanged)
/support, /onboarding	owned routes	owned routes (unchanged)

Any added/updated tests

Added src/apps/platform/src/routes/not-found/NotFound.spec.tsx, which renders the
platform home and fallback route shapes together and asserts that:

• /opportunities renders the not-found content with a link to /
• / still renders the home route, i.e. the catch-all does not shadow an owned route

yarn lint, npx tsc --noEmit and yarn build:dev all pass. yarn test:no-watch
reports 1171 passing tests with 24 failures in 19 suites; the identical 19 suites and 24
tests fail on an unmodified origin/dev checkout, so no new failures are introduced.

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