fix(signature-timeout-alert): derive parse message with useMemo instead of setState-in-effect

[otsimaofficial]

Problem

CI's Lint step has been failing on every merge to main since at least #283/#284/#287 (checked the last 5 CI runs on main — all red). The cause is ESLint's react-hooks/set-state-in-effect rule flagging SignatureTimeoutAlert.tsx: an effect that called setParseMessage(...) synchronously right after checking activeTransactionXdr.

This isn't related to any specific feature — it's blocking a clean CI signal for every future PR, including a batch of loading_spinner_skeleton issues (#275, #276, #278, #279) I'm opening alongside this one.

Fix

parseMessage is purely derived from activeTransactionXdr (parse it, capture the error message if parsing fails) — no external system, no subscription. Replaced the useState + useEffect pair with a single useMemo, computed synchronously during render instead of scheduling a second render via an effect. Behavior is unchanged; __tests__/signature_timeout_alert.test.tsx's waitFor assertion on the malformed-XDR case still passes (it now resolves on the first check instead of after an effect flush).

Testing

• npm run lint → 0 errors (previously 1 error / 4 warnings; the 4 pre-existing unrelated warnings remain).
• npx tsc --noEmit passes.
• npm run build passes.
• npm run test: full suite run: no failures traced to this file/change. The suite does have some pre-existing flakiness under load (a rotating subset of unrelated tests — multisig XDR fixtures, a couple of 5s test timeouts) that reproduces identically on a clean main checkout with this change stashed out, so it's unrelated to this fix.

🤖 Generated with Claude Code

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Hey @otsimaofficial! 👋 It looks like this PR isn't linked to any issue.

If this PR is for one of the issues assigned to you as part of a Wave, please link it to ensure your contribution is tracked properly. You can do this by adding a keyword to the PR description (e.g., Closes #123), or by clicking a button below:

Issue	Title
#276	Design empty list display views for loading_spinner_skeleton	Link to this issue
#279	Handle mobile viewports navigation styling in loading_spinner_skeleton	Link to this issue
#275	Implement responsive sizing layouts on loading_spinner_skeleton	Link to this issue
#278	Incorporate CSS micro-animations on loading_spinner_skeleton elements	Link to this issue

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