Use ISO-style calendar date formats

[keilogic]

Closes #847

Summary

• replace locale-dependent Moment picker formats with ISO-style date and 24-hour time formats
• render upcoming calendar event start/end times as YYYY-MM-DD HH:mm instead of Moment's localized calendar() output
• add focused tests for the picker format constants and the event card's visible date/time output

Tests

• Red baseline before the fix: npm run test -- --watch=false --progress=false --browsers=FirefoxHeadless --include src/app/calendar-app/calendar-formatting.spec.ts failed with TOTAL: 2 FAILED, 0 SUCCESS
• npm run test -- --watch=false --progress=false --browsers=FirefoxHeadless --include src/app/calendar-app/calendar-formatting.spec.ts
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• npm run policy (branch commits pass; command still prints historical upstream commit-message warnings)
• git diff --check origin/master...HEAD
• PowerShell equivalent of production build with SKIP_CHANGELOG=1 (node src/build/pre-build.js, npx ng build --configuration production --base-href=/app/ runbox7, node src/build/post-build.js) passed with build hash 662e16bea4b2816a

AI Disclosure

This PR was prepared with assistance from OpenAI Codex.