feat(links): surface source repo + Weblate links on site and in app

- site: set sourceURL (git.comunes.org/comunes/tane) and weblateURL
  (translate.comunes.org/projects/tane) — activates footer + collaborate links
- app About screen: add 'Source code' and 'Help translate' link tiles
- i18n: add about.sourceCode / about.translate / about.translateSubtitle
  (en base + es/pt/fr/de/ast; ja falls back to base), regenerate slang
- docs: add TRANSLATIONS.md runbook for the Weblate app component + git
  token + slang-regeneration workflow

apps/app_seeds/lib/i18n/fr.i18n.json

@@ -147,6 +147,9 @@
     "license": "Licence",
     "licenseValue": "AGPL-3.0",
     "website": "Site web",
+  "sourceCode": "Code source",
+  "translate": "Aider à traduire",
+  "translateSubtitle": "Aidez à traduire Tane dans votre langue",
     "openSourceLicenses": "Licences open source",
     "openSourceLicensesSubtitle": "Bibliothèques tiers et leurs licences",
     "copyright": "© {years} Association Comunes, sous AGPLv3"