feat(i18n): add internationalization infrastructure

[Ridanshi]

Closes #1082

Architecture decision

DevTrack uses the Next.js App Router with a single root layout, client providers, custom auth/rate-limit middleware, and Supabase-backed user settings. next-intl was selected because it fits App Router server/client rendering, supports request-scoped message loading, and avoids a disruptive route-prefix migration.

Locale strategy

Supported locales are English (en) and Spanish (es). Routes remain unprefixed for this foundation. Locale metadata includes direction so RTL languages can be added later without changing the provider shape.

Persistence strategy

Authenticated language preference is stored as users.preferred_locale through the existing settings API and mirrored into a devtrack-locale cookie. The cookie lets pre-auth and server-rendered pages pick the active locale quickly, while the database setting survives refresh, login, and other devices once settings are loaded.

Fallback behavior

Locale resolution priority is user preference, then cookie, then browser Accept-Language, then English. Message loading merges locale messages over English so missing translated keys fall back to English and rendering does not break.

Supported languages

• English
• Spanish

Files changed

• Added next-intl config, request helpers, locale metadata, detection, and lookup helpers.
• Added messages/en.json and messages/es.json.
• Wired the root layout and middleware for locale context and detection.
• Added preferred_locale persistence to settings API and migration.
• Added the Settings language dropdown.
• Externalized focused navigation, dashboard, settings, auth, achievement, common button, and common label strings.

Tests added

• Locale detection priority and fallback tests.
• Translation lookup, Spanish rendering, and missing-key behavior tests.
• Settings API locale persistence and validation tests.
• Dashboard header provider coverage update.

Verification commands

• corepack pnpm lint
• corepack pnpm typecheck (not defined; repo uses type-check)
• corepack pnpm type-check
• corepack pnpm test test/i18n.test.ts test/user-settings-api.test.ts test/useUserSettings.test.tsx test/components/DashboardHeader.test.tsx
• corepack pnpm test

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

This PR uses pnpm-lock.yaml and pnpm-workspace.yaml, but the project uses npm (package-lock.json). This would break npm ci in CI. Please switch to npm and regenerate the lock file, and also confirm no existing routes break with the locale routing change.

[Priyanshu-byte-coder]

CI is failing on this PR (Type check, Lint, Build). Please fix the errors and push an update.