ci(port,i18n): pull translations from Transifex directly

[scarf005]

Purpose of change (The Why)

Similar to CleverRaven/Cataclysm-DDA#84375. Currently, translation files are committed to the repository and updated daily via automated workflow. This causes:

• Repository bloat from daily translation update commits
• Translation lag (releases only get translations from last daily pull, not latest)
• Unnecessary git history noise

Describe the solution (The How)

Port DDA's approach: pull translations from Transifex during CI builds instead of daily commits.

New workflow (.github/workflows/build-translations.yml)

• Pulls translations from Transifex during CI builds
• Validates .po files and generates statistics
• Uploads translations as artifacts

Updated workflows

• release.yml - Added build-translations as dependency
• build.yml - Downloads translation artifacts before building (with continue-on-error for forks)

How 2 Translation

• lang/update_stats.sh - Generates translation statistics
• lang/discard_invalid_po.sh - Validates .po files from Transifex
• lang/po/placeholder.po - Minimal .po file for builds without Transifex access
• lang/CMakeLists.txt - Updated to skip placeholder.po

Documentation

• Added guide in building docs explaining how to build with translations locally

Describe alternatives you've considered

• Keep current system: repo bloat noooooooooooo
• Store translations separately: could work but it's easier to just tx pull

Testing

tested on https://github.com/scarf005/Cataclysm-BN/actions/runs/21029703506

verified artifact contains:

• lang/po/ with 25 translation files (ar, da, de, el, es_AR, es_ES, fr, hu, id, is, it_IT, ja, ko, nb, nl, pl_PL, pt_BR, ru_RU, sr, tr, uk, uk_UA, zh_CN, zh_TW)
• src/lang_stats.inc with translation statistics

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

Optional

• This PR ports commits from DDA or other cataclysm forks.
  • I have added port scope to the PR title.
  • I have attributed original authors in the commit messages adding Co-Authored-By in the commit message.
  • I have linked the URL of original PR(s) in the description.
• This is a PR that modifies build process or code organization.
  • Please document the changes in the appropriate location in the docs/ folder.
  • If documentation for this feature or process does not exist, please write it.
  • If the change alters versions of software required to build or work with the game, please document it.