Add tests for useDebounce's timer cleanup

[Abidoyesimze]

Summary

Small but genuinely untested timer logic: stale-timer cleanup on rapid value/delay changes is exactly the kind of race that regresses silently without a test.

Affected files

• src/hooks/useDebounce.ts

Tasks

• Add a test asserting rapid value changes reset the pending timer instead of firing multiple updates.
• Add a test asserting a changed delay is respected on the next update.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

testing, good first issue

Notes

Sourced from the 2026-07-25 round-2 follow-up audit of the frontend codebase at commit 779e531, run after the first 60-issue audit batch (#525-#584) was substantially resolved. Verified against current file content at time of writing.