🐛 fix(i18n): fix untranslated texts and remove unused translation keys

[shurco]

Changes made in this commit:

• Modified: web/admin/src/lib/components/Pagination.svelte
• Modified: web/admin/src/lib/components/cart/View.svelte
• Modified: web/admin/src/lib/components/form/Select.svelte
• Modified: web/admin/src/lib/components/page/Seo.svelte
• Modified: web/admin/src/lib/components/payment/Paypal.svelte
• Modified: web/admin/src/lib/components/payment/Spectrocoin.svelte
• Modified: web/admin/src/lib/components/payment/Stripe.svelte
• Modified: web/admin/src/lib/components/product/Digital.svelte
• Modified: web/admin/src/lib/components/product/Seo.svelte
• Modified: web/admin/src/lib/components/product/View.svelte
• Modified: web/admin/src/lib/components/setting/Letter.svelte
• Modified: web/admin/src/lib/composables/usePaymentSettings.ts
• Modified: web/admin/src/lib/layouts/Main.svelte
• Modified: web/admin/src/lib/utils/apiHelpers.ts
• Modified: web/admin/src/lib/utils/costFormat.ts
• Modified: web/admin/src/routes/carts/+page.svelte
• Modified: web/admin/src/routes/install/+page.svelte
• Modified: web/admin/src/routes/pages/+page.svelte
• Modified: web/admin/src/routes/products/+page.svelte
• Modified: web/admin/src/routes/settings/auth/+page.svelte
• Modified: web/admin/src/routes/settings/mail/+page.svelte
• Modified: web/admin/src/routes/settings/main/+page.svelte
• Modified: web/admin/src/routes/settings/payment/+page.svelte
• Modified: web/admin/src/routes/settings/socials/+page.svelte
• Modified: web/admin/src/routes/settings/webhook/+page.svelte
• Modified: web/admin/src/routes/signin/+page.svelte
• Modified: web/site/src/lib/components/Footer.svelte
• Modified: web/site/src/lib/components/Header.svelte
• Modified: web/site/src/lib/components/NotFoundPage.svelte
• Modified: web/site/src/lib/components/Overlay.svelte
• Modified: web/site/src/lib/components/Pagination.svelte
• Modified: web/site/src/lib/components/ProductCard.svelte
• Modified: web/site/src/routes/+error.svelte
• Modified: web/site/src/routes/+page.svelte
• Modified: web/site/src/routes/[slug]/+page.svelte
• Modified: web/site/src/routes/cart/+page.svelte
• Modified: web/site/src/routes/cart/payment/cancel/+page.svelte
• Modified: web/site/src/routes/cart/payment/success/+page.svelte
• Modified: web/site/src/routes/products/[slug]/+page.svelte
• Modified: README.md
• Added: web/admin/src/lib/i18n/index.ts
• Added: web/admin/src/lib/i18n/locales/en.json
• Added: web/admin/src/lib/i18n/locales/zh.json
• Added: web/site/src/lib/components/LanguageSwitcher.svelte
• Added: web/site/src/lib/i18n/index.ts
• Added: web/site/src/lib/i18n/locales/en.json
• Added: web/site/src/lib/i18n/locales/zh.json

Key changes:

• Remove unused translation keys from admin and site locale files
• Fix untranslated texts in admin panel (mail settings, payment settings, etc.)
• Fix hardcoded "Loading..." text in Digital component
• Fix message priority in apiHelpers to use translated messages over server messages
• Add missing translations for letter legend items (Site name, Amount of payment, etc.)
• Add missing translations for popup messages (Status updated, Data saved, etc.)
• Update README.md with multi-language support documentation

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.
Explain the details for making this change. What existing problem does the pull request solve?

Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)
• I tried to make my code as fast as possible with as few allocations as possible
• For new code I have written benchmarks so that they can be analyzed and improved

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