fix(hooks): guard freighter API behind window check for SSR (#010)

[Clement-coder]

Summary

/offramp was crashing on SSR because useFreighter called into the Freighter
browser extension before window existed. Guarding getFreighterApi() behind
typeof window !== 'undefined' makes the hook return a stable
{ isConnected: false, publicKey: null } during static render, eliminating the
crash and the hydration diff.

Linked issue

Closes #10

Changes

• hooks/useFreighter.ts — getFreighterApi() throws early when
  typeof window === 'undefined'; the checkInstalled effect catches it and stays at
  INITIAL_STATE, giving zero hydration diff

Testing notes

Automated

• npm run typecheck · ⏳ not run / ✅ green / ❌ failing
• npm run lint · ⏳ not run / ✅ green / ❌ failing
• npm run test · ⏳ not run / ✅ green / ❌ failing
• npm run build · ⏳ not run / ✅ green / ❌ failing

New / modified tests

• N/A — one-line SSR guard; no client behaviour changed

Manual verification

• N/A — change only affects the server-render path; no Freighter interaction
  involved

Screenshots / recordings

N/A — no user-visible change; fixes a server-side crash.

Checklist

Correctness

• The PR title follows Conventional Commits (auto-linted)
• One logical change; unrelated cleanup was split into a separate PR
• npm run typecheck passes
• npm run lint passes with zero new warnings
• npm run test passes; new behaviour has a test
• npm run build passes

Data integrity

• No fabricated rates, stub prices, or placeholder exchange rates
• No isMock, // MOCK, // TODO: replace with real data, or commented-out real
  code
• Not touching an anchor, SEP-10, SEP-24, or the status poll

Security & non-custody

• No new code path holds user keys, user funds, or long-lived anchor JWTs
• Every signing action is performed by the user's wallet (Freighter today)
• No secrets committed; .env.local is unchanged; no new env vars

Docs

• No user-facing behaviour change — no CHANGELOG.md entry needed
• No API / schema / architecture change
• No new env var

Release hygiene

• No wave deliverable touched
• No dependency added
• No breaking change

Breaking changes

None.

For reviewers

Single-line fix. Guarding at getFreighterApi() covers both the mount effect and
the connect callback in one place, rather than duplicating the check in each
useEffect

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