fix(tests): stop faking timezone coverage on the vitest threads pool

[h4yfans]

Closes #1955

Problem

The renderer vitest project runs on vitest's threads pool, where process.env is a per-worker copy: assigning process.env.TZ at runtime never reaches the C++ tzset, so the process stays pinned to the host's zone regardless of what a test sets. journal-template-resolution.test.ts parameterised a describe('timezone', ...) block over three zones this way and passed tautologically — every case ran in the host zone.

Digging further: even with a working TZ knob, that test's coverage would still have been illusory. resolveJournalTemplateId's only timezone-sensitive step is parseISODate, which builds the Date from the string's Y/M/D parts (not a UTC parse), so the weekday it reads back is identical in every zone by construction — that invariance is the whole point of parseISODate over new Date(isoDate).

Changes

• apps/desktop/src/renderer/src/lib/journal-template-resolution.test.ts — deleted the illusory in-process TZ matrix. Replaced with a genuine child-node test (the real coverage worth keeping): parseISODate reads 2026-08-17 as Monday in UTC, America/Los_Angeles, and Pacific/Kiritimati, proving the weekday stays correct across zones rather than shifting the way a UTC parse would.
• apps/desktop/src/renderer/src/lib/test-support/run-in-tz.ts (new) — shared helper extracted from the two existing genuine child-process TZ tests (local-day-range.test.ts, journal-day-panel-day-range.test.tsx), so this pattern has one owner instead of getting re-copied (or re-broken) per file.
• apps/desktop/src/renderer/src/lib/local-day-range.test.ts, apps/desktop/src/renderer/src/components/journal/journal-day-panel-day-range.test.tsx — switched to the shared helper, no behavior change.
• apps/desktop/src/renderer/src/lib/renderer-process-env-tz-is-inert.test.ts (new) — proves the trap directly: asserts that flipping process.env.TZ in-process does not change the observed UTC offset across four zones, and cross-checks that the same flip in a real child process does.

apps/desktop/src/renderer/src/hooks/use-today.test.tsx and local-day-range.test.ts already carried the correct comment about this wall (from #1953/#1964); journal-template-resolution.test.ts was the one file silently relying on the broken pattern, now fixed to match.

Verification

• pnpm --filter @memry/desktop typecheck:test — clean
• Targeted vitest run on all four touched/added test files — 38/38 passing
• pnpm exec eslint on the new non-test source file — clean

Docs gate: pushed with MEMRY_DOCS_IMPACT_SKIP=1 — test-support/run-in-tz.ts is internal test infrastructure only, no user-facing behavior or docs impact.

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit 453681a.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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