[FEATURE]: Land TUI i18n groundwork adapted from #48731 #51998

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Describe the enhancement you want to request

The TUI (packages/tui) currently ships zero i18n: roughly 700–1000 user-facing strings are hardcoded English, while the app/desktop side already has per-locale dictionaries.

#48731 by @neriousy laid down the full groundwork (lazy per-locale dictionaries, t()/plural()/number() helpers, language picker, config.language) and the architecture looked solid in review, but the PR has been stalled — no updates since 2026-09-23 and no response to the collaboration comment on the PR.

Rather than let that work rot, I would like to carry it forward:

1. Merge Feat/tui i18n #48731 onto the current v2 branch (it had fallen ~550 commits behind).
2. Resolve conflicts with v2's evolved TUI code (theme tokens, dialog APIs, diff-viewer sources, fail/attempt helpers) while keeping the PR's language.t(...) wiring.
3. Drop parts of the PR that reference APIs v2 has since removed.
4. Run lint/typecheck/tests with a baseline comparison to prove zero new failures.

All of @neriousy's original commits are preserved via a merge commit, so authorship stays intact.

Related: #48731

Follow-up PR: #52000.