test(issue-time): stabilize relative age fixtures across DST

[beruro]

Problem

The issue-age test creates “months ago” and “days ago” timestamps by changing local calendar dates. A daylight-saving transition can shift the elapsed duration by an hour, making the shared relative-time formatter correctly return 10mo ago while the test expects 11mo ago. This intermittently fails the full frontend CI suite, including the conversation Sources PR.

Solution

Build these test fixtures from elapsed milliseconds, matching the formatter's 24-hour day and 30-day month definitions. Production behavior is unchanged; the test now measures the intended duration in every time zone.

Potential risks

This changes only test fixture construction. It does not change user-visible date formatting, stored data, APIs, or runtime behavior. The full repository CI suite remains to be observed on this PR before merge; the targeted test passed in both a daylight-saving time zone and UTC.

Verification

• TZ=America/Los_Angeles pnpm test src/modules/WorkStation/CodeEditor/Panels/EditorPrimarySidebar/hooks/__tests__/workstationIssueHelpers.test.ts — 29 passed.
• TZ=UTC pnpm test src/modules/WorkStation/CodeEditor/Panels/EditorPrimarySidebar/hooks/__tests__/workstationIssueHelpers.test.ts — 29 passed.
• pnpm typecheck:fast — passed.
• Changed-file ESLint, Prettier check, commit hooks, and git diff --cached --check — passed.
• Full repository suite and native checks were not run locally. This test-only change has no visual UI state to capture.