feat: persist share chat language preference

[YYH211]

What

• Add an isolated share-chat language preference key so public share pages do not overwrite the main app language cookie.
• Add language switch entries for share chat and chat sidebar user menus.
• Preload alternate i18n resources for client-side language switching with reload fallback.

Why

Public share-chat visitors should be able to switch language without polluting the logged-in portal language preference.

Validation

• pnpm install --frozen-lockfile
• pnpm --filter @fastgpt/app typecheck
• git diff --cached --check
• git diff --check HEAD
• pre-commit lint-staged eslint/prettier

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	13.9%	1135 / 8161
🔵	Statements	13.89%	1190 / 8564
🔵	Functions	12.52%	246 / 1964
🔵	Branches	11.82%	531 / 4489

File Coverage

No changed files found.

Generated in workflow #439 for commit 156d4c0 by the Vitest Coverage Report Action

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