feat(mail): show attachment detail tooltips

[keilogic]

Closes #1446.

Summary

• Add an attachment tile tooltip and matching aria label with the full filename, content type, and formatted file size.
• Reuse the existing attachment metadata and size formatter, with fallbacks for missing filename, type, or size.
• Add focused SingleMailViewer coverage for the attachment detail tooltip.

Testing

• Confirmed the focused tooltip spec failed before the implementation because the attachment tile did not provide a MatTooltip directive.
• npm ci
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx ng test runbox7 --watch=false --browsers=FirefoxHeadless --include=src/app/mailviewer/singlemailviewer.component.spec.ts (9 success; existing template warnings and mocked attachment thumbnail 404 noise)
• npx ng test runbox7 --watch=false --browsers=FirefoxHeadless (first run hit a Firefox no-message disconnect after 245 success; rerun passed with 246 success and 2 skipped, with existing template warnings)
• npm run lint -- --quiet
• git diff --check origin/master...HEAD
• git show --check --stat --oneline HEAD
• npm run policy (current commits OK; historical upstream commit-message warnings still printed)
• $env:SKIP_CHANGELOG='1'; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js (production build hash 6494274c6d5a7d0c; existing Angular/Sass/CommonJS warnings)

AI Disclosure

I used OpenAI Codex/GPT-5 in a local development workspace to inspect the relevant code, implement this change, and run the validation commands listed above. I reviewed the resulting diff and validation output before submitting.