[Web][Backend] EN/TR i18n (consolidated): backend lang preference + error localization + full web translation sweep

[mustafa-5493]

📝 Description

Full-stack EN/TR i18n, consolidated from 10 stacked PRs into one branch for unified review. Closes #291, #292, #505, #546, #548, #550, #553, #557, #560, #562, #564, #574.

Backend

• Language enum + preferredLanguage column on RegisteredUser (default EN, V6 Flyway migration)
• Owner-only PUT /api/users/{id}/profile/language; field exposed in UserProfileDTO (self-view only, same privacy gate as email)
• MessageSource + AcceptHeaderLocaleResolver; EN + TR message bundles for user-facing errors

Web

• i18next + react-i18next + browser-language detector
• LanguageSwitcher (EN/TR pill) in the navbar at every breakpoint; persists to backend on login, falls back to localStorage for guests
• Accept-Language forwarded by apiFetch and every raw fetch (avatar / media / fix-request / routes)
• Translation sweep across Navbar, NotificationDropdown, MapSearchBar, Login, Signup, Home, OnboardingTutorial, Profile + UserProfile + AvatarUploader + RoutingPreferencesSection + CustomPresetEditorModal + MyReports + MyReportDetailModal + UserSearch, Feed + FeedLocationPickerModal + MapFilters + ReportObjectsList, ReportPanel + CreateReportPanel + CreateFixRequestPanel + ObjectTutorialModal, Leaderboard page + nav link, toasts, errors, aria-labels
• localizeObjectType(t, cfg) helper so object/issue/measurement labels switch from EN defaults
• Report titles rebuilt at render time from translated parts

Atomic commit history preserved , best reviewed via the Commits tab, not the whole diff.

📊 Type of Change

• New feature

✅ Acceptance Criteria

• EN/TR pill flips every translated surface
• Persists via PUT /api/users/{id}/profile/language, restores from /api/users/me.preferredLanguage on login
• Guests get browser-language detection + localStorage
• Backend errors localize via Accept-Language
• Visible on mobile
• 474 frontend tests pass; backend tests pass

🧪 How to Test

1. docker compose -f docker-compose.local.yml --env-file backend/.env up -d --build --force-recreate backend
2. cd frontend && npm run dev, log in
3. Click EN/TR in the navbar → UI flips
4. Reload → choice sticks
5. Log out, log in on a fresh profile → preference restores from backend
6. Trigger an auth error in TR → message comes back localized

⏱️ Estimated Review Time

15 min (commit-by-commit)