[honfox]

This adds Simplified Chinese as a UI language, plus on-demand translation of post descriptions and tags.

What changed:

• lib/shared/data/trans.dart: an English to Chinese dictionary, plus two helpers. t() translates a plain string, tf() translates a template containing {placeholders}. Strings without a translation fall back to the original English text.
• A language setting under Settings (follow system / English / Chinese), persisted with the existing settings backend.
• User facing strings across the app are wrapped in t()/tf().
• lib/shared/data/translate.dart: translates post descriptions and tags on demand. It uses the public Google Translate endpoint and falls back to MyMemory. Results are cached in memory. No new dependencies, dio was already used.
• Fixed a UTF-8 BOM at the start of lib/app/widget/app.dart. Current Dart rejects it, so flutter build apk --release fails with "The non-ASCII space character U+FEFF can only be used in strings and comments".
• Small dependency bumps in pubspec.lock, and analyzer excludes for the platform folders.

Most of the diff is the mechanical string wrapping. The actual logic changes are in the two new files, the settings page, and the post description and tag widgets.

Tested with flutter build apk --release, which succeeds.