fix(frontend): localize dashboard

[Optic00]

Summary

• Localize dashboard headings, widgets, customization controls, and state messages across every supported locale.
• Format relative timestamps with the active application locale.
• Keep user-customized section text unchanged while translating untouched built-in sections.
• Preserve canonical defaults when translated section text is opened and saved without edits.
• Validate metadata-driven translation keys in the i18n checker.

Verification

• npm run check
• npm run typecheck - 0 errors and 0 warnings
• npm run test:run - 7 tests passed
• npm run build
• Browser E2E against a Docker deployment: save an unchanged German default section, switch German to English and back, and verify the heading follows the active locale.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[Optic00]

I have read the CLA Document and I hereby sign the CLA

[stefan-ernst]

@Optic00 - thanks a lot for the PR. One observation:

const metadataMatches = content.matchAll(
      /\b(?:name|description|title|subtitle)Key:\s*['"]([a-zA-Z][a-zA-Z0-9_.]+)['"]/g
    );

does not match the labelKey introduced later in the PR, is this intentional?

[Optic00]

Good catch, this wasn't intentional. check-i18n.js only scanned for nameKey/descriptionKey/titleKey/subtitleKey, so labelKey references were silently skipped. I added labelKey to the scanner and pushed the fix in 3ed62c88.

While addressing that, I also fixed the relative-time wording (numeric: 'always' so "1 week ago" no longer becomes "last week") and replaced the userFallback string with per-locale salutation templates. Frontend checks, the Vitest suite, and check-i18n all pass locally.

[stefan-ernst]

all good!