feat(i18n): add Turkish locale

[sercanarikan]

What does this PR do?

Adds Turkish (tr) language support across the Multica product UI, landing page, onboarding content, and documentation.

This is a clean replacement for the previously closed PR #3140, based directly on the current main branch and contained in a single feature commit.

Related Issue

Closes #3145

Changes Made

• Added Turkish to supported application locales and backend language validation.
• Added Turkish UI locale bundles with current locale parity.
• Added Turkish Preferences, landing page, onboarding, and helper content.
• Added Turkish documentation and README.
• Added Turkish HTML language mapping and onboarding runtime copy.

How to Test

• pnpm --filter @multica/views exec vitest run locales/parity.test.ts
• pnpm typecheck
• Select Turkish from Settings → Preferences → Language.
• Open the Turkish documentation routes under /docs/tr.

Verification

• Locale parity: 212/212 passed.
• TypeScript typecheck: passed.
• Branch is based on current main and contains one feature commit.

Supersedes #3140.

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