add multilingual support with language switcher and i18n integration

[secoro]

Closes #30

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 2.83019% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.78%. Comparing base (9e2b919) to head (55de9a2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
frontend/src/i18n/index.tsx	0.00%	33 Missing ⚠️
frontend/src/pages/MealPlanner.tsx	0.00%	12 Missing ⚠️
frontend/src/components/modals.tsx	0.00%	10 Missing ⚠️
frontend/src/pages/auth.tsx	0.00%	10 Missing ⚠️
frontend/src/components/LanguageSwitcher.tsx	0.00%	8 Missing ⚠️
frontend/src/App.tsx	0.00%	7 Missing ⚠️
frontend/src/components/RecipeDetailModal.tsx	0.00%	6 Missing ⚠️
frontend/src/components/RecipesSection.tsx	0.00%	6 Missing ⚠️
frontend/src/components/InventorySection.tsx	0.00%	5 Missing ⚠️
frontend/src/components/AccountMenu.tsx	0.00%	2 Missing ⚠️
... and 4 more
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Additional details and impacted files

@@             Coverage Diff              @@
##               main      #31      +/-   ##
============================================
- Coverage     27.76%   26.78%   -0.98%     
  Complexity      109      109              
============================================
  Files            49       54       +5     
  Lines          1646     1706      +60     
  Branches        305      313       +8     
============================================
  Hits            457      457              
- Misses         1147     1207      +60     
  Partials         42       42              

Flag	Coverage Δ
backend	50.93% <ø> (ø)
frontend	2.47% <2.83%> (-0.19%)	⬇️

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