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

site/config.toml

@@ -17,8 +17,8 @@ enableGitInfo = false
   # Links rendered only when set, so nothing broken shows before they exist:
   playStoreURL = ""   # Google Play, once published
   fdroidURL = ""      # F-Droid, once published
-  sourceURL = ""      # public source repository (AGPL) — set when available
-  weblateURL = ""     # translation platform — set when hosting is ready
+  sourceURL = "https://git.comunes.org/comunes/tane"      # public source repository (AGPL)
+  weblateURL = "https://translate.comunes.org/projects/tane/"     # translation platform
 
 [languages]
   [languages.en]