Fix missing alternative text for event metadata icons

[Copilot]

Summary

• add accessible text labels for event metadata icons on the Event detail page
• ensure location, date/time, online link, and registrants icons are exposed to screen readers
• reuse existing localized messages in EventsShowPage/messages.ts

Validation

• cd /home/runner/work/citizenlab/citizenlab/front && npm run lint (fails in this sandbox: eslint: not found because dependency installation is blocked)
• cd /home/runner/work/citizenlab/citizenlab/front && npm run build:test (fails in this sandbox: cross-env: not found for the same reason)
• cd /home/runner/work/citizenlab/citizenlab/front && npm test -- --runInBand --watch=false app/containers/EventsShowPage (fails in this sandbox: jest: not found for the same reason)
• parallel validation run: Code Review ✅ (1 non-blocking import-order style suggestion), CodeQL ✅ (0 alerts; JS DB skipped due size)

[EdwinKato]

@copilot fix this issue where the front-extract-intl step is failing. See some context below and note not to make the same mistake in future

cl2-front@1.0.0 extract-intl
tsc --skipLibCheck app/containers/App/constants && babel-node --presets @babel/env -- ./internals/scripts/extract-intl.js

Storing language files in memory ✓
Run extraction on all files
Run extraction on all files
.Error transforming file: app/containers/EventsShowPage/messages.ts
Error: Duplicate definition found for id 'app.containers.EventsShow.eventDateTimeIcon'
Some messages.js files contain errors. First fix them and run the script again.
Exited with code exit status 1

[Copilot]

@copilot fix this issue where the front-extract-intl step is failing. See some context below and note not to make the same mistake in future

cl2-front@1.0.0 extract-intl
...

Fixed in 660d26f (with cleanup in 0454be7). I removed the duplicate intl IDs by changing the new EventsShowPage icon message IDs to unique app.containers.EventsShowPage.* values, so they no longer collide with EventCards IDs during extract-intl.

Screenshot: N/A — this is an internal i18n ID fix only and does not produce a visual UI change.

[cl-dev-bot]

	Warnings
⚠️	The changelog is empty. What should I put in the changelog?
⚠️	The PR title contains no Jira issue key (case-sensitive)
⚠️	The branch name contains no Jira issue key (case-sensitive)

	Messages
📖	Run the e2e tests
📖	Check translation progress

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