Close remaining empty/error/loading gaps

[james2177]

Summary

• ActivityFeed silently rendered nothing when the feed request failed or genuinely had no items — isLoading only covered the initial fetch. Now distinguishes a "Live feed unavailable right now." error state from a "No fills yet." empty state, and still shows cached items if a later background refresh errors rather than replacing them with an error box.
• IntentDetailPage rendered a blank card area when a request succeeded but returned no intent data, distinct from the network/404 error branch (!intent ? null : ...). Adds a "No details found for this intent." state.
• ConnectWalletButton's failed-connect state only surfaced its error via a title tooltip — invisible to touch/keyboard users and easy to miss. Direct connect attempts now also fire a toast via the PR Wire solver leaderboard to real API #16 system. This is scoped to the component's own click handler rather than inside the wallet store's connect(), so it doesn't double-toast when useSwapSubmission/useAcceptIntent/useSolverRegistration call connect() internally — those already handle and toast their own errors.
• Adds a branded src/app/not-found.tsx (Nav/Footer + a link home) in place of Next's default 404 page.

Test plan

• npm run lint
• npm test (9 new tests: ActivityFeed empty/error/cached-on-error, IntentDetailPage not-found, ConnectWalletButton toast-on-failure, NotFound render + main landmark; 139 total passing)
• rm -rf .next && npm run build
• Manually verified npm run dev returns a 404 status with the branded "Page not found" copy for an unmatched route

Branched off main (PR #18 already merged).