Resolve all 10 upstream v1.21.1 merge conflicts into custom, keeping
custom's GitLab/Gitea forge customizations layered on upstream while
lifting upstream improvements. Restored tr.ts i18n key parity with the
custom en.ts dictionary (471 custom forge keys added, en fallback) so the
upstream-introduced locale stays in parity.
Also stage bun.lock version alignment (1.21.0 -> 1.21.1) that matches the
staged package.json bump.
Verification: ui + web type-check pass; ui/web tests pass except
pre-existing failures on the custom baseline (forge.test.ts, session-actions,
issue-1637-2270, routes.test.js fs-stat directory scope).