fix: dead Log In button, missing rel, window.location nav, missing AbortController

[MarvyNwaokobia]

Summary

Fixes four independent frontend issues in a single PR:

Closes #560
Closes #561
Closes #562
Closes #563

• [Frontend] Non-functional 'Log In' button in Navbar #562 — Remove dead Log In button from Navbar: The Log In button had no onClick or href and was never wired to any auth flow. WalletButton is the sole auth entry point, so the orphaned button is removed.
• [Frontend] Add rel=noopener noreferrer to external explorer link in ActivityHistory #560 — Add rel="noopener noreferrer" to Stellar Expert link in ActivityHistory: The external explorer anchor had target="_blank" without rel, exposing the app to reverse-tabnabbing. Fixed in ActivityHistory.tsx. A grep confirmed all other target="_blank" links in the codebase already carry the correct rel.
• [Frontend] NotificationDropdown 'View All Activity' uses window.location.href instead of router navigation #561 — Replace window.location.href with router.push in NotificationDropdown: The "View All Activity" button was forcing a full-page reload, discarding SPA state (wallet context, React Query cache, SSE connection). Now uses useRouter().push('/activity') from next/navigation and closes the dropdown before navigating.
• [Frontend] Activity page fetch has no AbortController cleanup #563 — Add AbortController cleanup to activity page fetch: fetchActivity inside the useEffect had no abort signal, so rapidly switching tabs or unmounting mid-flight left stale in-flight requests that could race setEvents/setHasMore. An AbortController is now created in the effect, its signal passed to fetch, and abort() called on cleanup. AbortError is silently ignored.

Files changed

• frontend/src/components/Navbar.tsx
• frontend/src/components/dashboard/ActivityHistory.tsx
• frontend/src/components/NotificationDropdown.tsx
• frontend/src/app/activity/page.tsx

Test plan

• Verify Navbar no longer shows a Log In button; WalletButton still renders correctly
• Open ActivityHistory, click the external explorer link — confirm it opens in a new tab without exposing window.opener
• Open NotificationDropdown, click "View All Activity" — confirm it navigates via the Next.js router (no full reload), and the dropdown closes
• On the activity page, rapidly switch tabs — confirm no stale-response race conditions or console warnings about state updates after unmount

[MarvyNwaokobia]

Closing in favor of a new PR with proper issue linking.