feat(i18n): add Portuguese (pt-BR) language support to entire frontend

[PauloHenriqueJr]

Add complete internationalization infrastructure and Portuguese translations covering all pages, components, and UI strings in the Paperclip frontend.

What's included

• New LanguageContext with automatic browser locale detection and localStorage persistence
• New locale.ts with t() translation function and ~1450 en/pt-BR translation entries
• Language toggle (EN/PT-BR) in the bottom bar of the layout
• All 27+ pages translated (Dashboard, Issues, Agents, Skills, Settings, etc.)
• All 60+ components translated (Sidebar, CommandPalette, dialogs, forms, cards, etc.)
• Status labels, agent roles, dates, and currency formatted per locale
• Migrated all legacy isPt ? "..." : "..." patterns to centralized t() calls

How it works

• t("English key") returns the Portuguese translation when pt-BR is active, or falls back to English
• Language preference is auto-detected from browser locale and persisted in localStorage
• Components subscribe to language changes via useLanguage() hook for reactive re-renders

Files not translated (intentional)

• DesignGuide.tsx — internal dev component showcase, not user-facing

Thinking Path:

• Paperclip orchestrates AI agents for zero-human companies
• The UI subsystem only supports English, limiting adoption in non-English markets
• Portuguese-speaking users see a mix of translated and untranslated strings, creating
  inconsistent UX
• A centralized i18n system with full pt-BR coverage addresses this gap
• This pull request adds a complete translation infrastructure with ~1450 keys
• The benefit is enabling Portuguese-speaking teams to use Paperclip natively, and establishing
  the pattern for future language additions

What Changed:

• New ui/src/lib/locale.ts — centralized t() translation function with ~1450 en/pt-BR key
  pairs
• New ui/src/context/LanguageContext.tsx — React context for language state, browser detection,
  and localStorage persistence
• All 27+ pages updated to use t() for user-visible strings
• All 60+ components updated — migrated inline isPt ? "pt" : "en" patterns to t() calls
• ui/src/lib/utils.ts and ui/src/lib/timeAgo.ts updated for locale-aware formatting
• Language toggle button added to Layout footer (EN / PT-BR)

Verification:

• TypeScript compiles cleanly (tsc --noEmit — zero errors)
• All 642 existing tests pass (no regressions)
• Production build succeeds in ~9.5s
• Manually verified all major pages in both EN and PT-BR
• Language persists across page refreshes via localStorage
• Browser locale auto-detection works (Portuguese browser → PT-BR default)

Risks:

• Translation keys use English strings as keys — a typo in a key falls back to displaying the
  English string (safe degradation, never blank)
• Components without useLanguage() hook won't re-render on language switch until next navigation
  (acceptable since language changes are infrequent)
• DesignGuide.tsx intentionally not translated (internal dev tool)

Checklist

• I have included a thinking path that traces from project context to this change
• I have run tests locally and they pass
• I have added or updated tests where applicable
• If this change affects the UI, I have included before/after screenshots
• I have updated relevant documentation to reflect my changes
• I have considered and documented any risks above
• I will address all Greptile and reviewer comments before requesting merge

[greptile-apps]

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