feat(wallet): poll 2s/30s then focus+visibilitychange for mid-session Freighter install detection (#41)

[collinsezedike]

Summary

• Adds a 2s polling phase for the first 30s after mount so a mid-session Freighter install is surfaced within 2s
• After 30s, clears the interval and attaches window focus + document visibilitychange listeners instead (zero-cost ongoing detection)
• WatchWalletChanges subscription runs in parallel for account/network change tracking — the two concerns are independent
• 7 new tests in tests/freighter-detect.spec.tsx covering early-poll detection, event-driven detection, and cleanup

Test plan

• Open the page without Freighter installed — "Install Freighter" link shown
• Install Freighter extension — Connect button appears within ~2s without a page reload
• Wait 30s, then re-focus the tab after installing — Connect button still appears
• Unmount/navigate away — confirm no console errors about state updates on unmounted component

Closes #41

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[ezedike-evan]

The implementation is exactly right: 2s early polling for the first 30s, then zero-cost event-driven detection via `focus` + `visibilitychange`, with `WatchWalletChanges` running in parallel for account/network changes. The three-timer cleanup in the return function is correct (early interval, early timeout, watcher, fallback interval, and both event listeners all removed). `onVisibilityDetect` is properly hoisted outside `init()` so `removeEventListener` matches the same reference. The 7 new tests in `freighter-detect.spec.tsx` cover every path including the "no polls after 30s without an event" invariant.

Ready to merge — but this branch is stacked on #261 and cannot land until #261 is rebased and merged first. Once #261 is merged into `main`, rebase this branch on the updated `main` and push — it will merge cleanly.

```bash

After #261 is merged:

git fetch origin
git rebase origin/main
git push --force-with-lease
```

[ezedike-evan]

Approved and ready to land — waiting on #261 to merge first since this branch is stacked on it.

Once #261 is merged into `main`, rebase this branch on the updated `main` and push:

```bash
git fetch origin
git rebase origin/main
git push --force-with-lease
```

No code changes needed — just the rebase.