[mskdylan]

Summary

Test-only PR that adds deterministic regression tests for four hook-level issues in one coordinated change. No production code was modified — every test asserts the hooks' existing documented behavior and error contracts. All tests use Vitest fake timers and controlled clocks (no real sleeps).

#1783 — SSE reconnect timer cleanup (frontend/src/__tests__/useStreamEvents.test.tsx)

• Unmounting while a reconnect backoff timer is pending clears the timer (vi.getTimerCount() → 0) and no new EventSource is ever created, even after advancing the clock far past the delay.
• Unmounting one tick (1 ms) before the retry delay elapses leaves no timers and no reconnect.
• Unmount closes the active connection; events emitted on the stale source afterwards are dropped.
• A burst of repeated error events in the same tick schedules exactly one reconnect timer (no stacking), and the grown backoff delay is honored.
• After the reconnect-attempt cap (20) is exhausted, no timers remain scheduled.
• reconnecting/connected/error state is correct while a retry is pending and cleans up on unmount.

#1784 — Invalid decimal settings fallback (frontend/src/hooks/useSettings.test.ts)

• Missing or empty stored flowfi-decimal-places falls back to the documented default of 7.
• Supported values 2/4/7 hydrate and format at exactly the requested precision (boundaries covered).
• Fractional stored values (4.9, 2.7) are truncated to their integer prefix per parseInt semantics; leading zeros and surrounding whitespace are tolerated.
• Out-of-range values (12, 100) stay parseable but formatting never exceeds the 7 fractional digits that exist on-chain.
• Negative values (-2) never throw; malformed values (abc, seven, NaN, —) yield NaN and degrade to integer-only formatting without throwing.
• The hook hydrates malformed/out-of-range values without crashing, and a valid value set afterwards is applied (recovery). Empty currency/format values fall back to USD/full.

#1781 — Streaming amount pause transitions (frontend/src/hooks/useStreamingAmount.test.tsx)

• Initial state is 0 at stream start and accrues normally.
• Pausing freezes the value, removes the frame loop (0 timers); resuming restores accrual from the pre-pause value with exactly one frame loop (no duplicate timers).
• Repeated pause/resume cycles exclude paused time via totalPausedDuration and keep timer counts consistent.
• Boundary cases: stream paused at its start time stays 0; zero-rate and fully-withdrawn streams never schedule a frame loop.
• Clock drift: a 60 s wall-clock jump does not affect frame-driven accrual; the visibilitychange resync corrects the value to true elapsed time without duplicating timers.
• Unmount while streaming cancels the frame loop (cancelAnimationFrame, 0 timers) and no stale ticker mutates state; unmount while paused leaves nothing scheduled.

#1782 — Malformed SSE event recovery (frontend/src/__tests__/useStreamEvents.test.tsx)

• Malformed JSON, plain text, and empty payloads on subscribed event types are silently ignored (no crash, no error state, connection stays healthy, no reconnect timers scheduled).
• Valid events delivered after malformed payloads are processed normally.
• Unknown event types never reach the hook (transport-level contract) and do not affect processing.
• Partially delivered frames are treated as malformed per the parser contract; interleaved malformed payloads never corrupt the event list (order and contents verified).

Tests executed (actual results)

Command (in frontend/)	Result
npx vitest run src/__tests__/useStreamEvents.test.tsx src/hooks/useSettings.test.ts src/hooks/useStreamingAmount.test.tsx src/__tests__/useStreamingAmount.test.tsx	✅ 4 files, all tests passed (33 new tests added)
npm run test (full frontend suite, re-run on this branch rebased on main)	✅ 80 files, 768/768 tests passed
npm run test:coverage	✅ passed, thresholds met
npm run lint	✅ 0 errors (18 pre-existing warnings, none in touched files)
npx tsc --noEmit	✅ clean
npm run build (Next.js)	✅ passed

No existing tests were weakened, skipped, or deleted; no flaky real-time waits were introduced.

Closes #1781
Closes #1782
Closes #1783
Closes #1784