Fix Cypress failures: stub replag script, format ZIM date in UTC

[audiodude]

Problem

CI's frontend-cypress job started failing on every spec (13 of 13, 62 failures) with:

TypeError: Failed to execute 'setHTML' on 'Element': Failed to read the 'sanitizer' property from 'ElementSetHTMLOptions': Failed to convert value to 'Sanitizer' — at https://tools-static.wmflabs.org/replag-embed/replag-embed.js:41

The replag banner script is fetched live from wmflabs on every page load (index.html). An upstream change to it (it now passes a new-spec Sanitizer config to Element.setHTML()) throws in Cypress's bundled Electron browser, and Cypress fails any test whose app throws an uncaught exception. Nothing in this repo changed — main was green two days before the same code went red.

Fixes

• Stub replag-embed.js in cypress/support/e2e.js: a global cy.intercept serves an empty body for the script, so tests no longer execute third-party code fetched over the network (also removes a flakiness/external-dependency liability). No test asserts on the banner. Real browsers are unaffected.
• Format the ZIM next-generation date in UTC (ZimFile.vue): found while re-running the suite locally. The backend sends a date-only YYYY-MM-DD string, which new Date() parses as UTC midnight, but it was formatted in the viewer's local timezone — so the zimFile spec's "shows the next generation date" test fails in any timezone behind UTC, and real users west of UTC see the previous day. Adding timeZone: 'UTC' to toLocaleDateString fixes both. (This never failed in CI, which runs in UTC.)

Testing

Full Cypress suite run locally against the dev stack, twice: 193/193 passing across all 13 specs.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.64%. Comparing base (cb7b6f4) to head (4d38e8e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1214   +/-   ##
=======================================
  Coverage   92.64%   92.64%           
=======================================
  Files          78       78           
  Lines        4987     4987           
=======================================
  Hits         4620     4620           
  Misses        367      367           

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