Update: add Chinese README and frontend localization

[SI-RUI-ZHANG]

Original issue
Skill Manager needs a public Simplified Chinese README and bilingual frontend UI.

Fix
Adds first-party frontend i18n, a persisted sidebar language selector, Chinese UI copy, protected product terms, and README.zh-CN.md.

Changed files
README.md
README.zh-CN.md
frontend/src/App.test.tsx
frontend/src/App.tsx
frontend/src/app/capability-registry/overview.ts
frontend/src/app/capability-registry/sidebar.ts
frontend/src/components/BulkActionBar.tsx
frontend/src/components/ConfirmActionDialog.tsx
frontend/src/components/FilterBar.tsx
frontend/src/components/Sidebar.tsx
frontend/src/features/marketplace/api/mcp-queries.ts
frontend/src/features/marketplace/components/CliMarketplaceCard.tsx
frontend/src/features/marketplace/components/CliMarketplaceDetailSheet.tsx
frontend/src/features/marketplace/components/CliMarketplaceDetailView.tsx
frontend/src/features/marketplace/components/MarketplaceCard.tsx
frontend/src/features/marketplace/components/MarketplaceDetailSheet.tsx
frontend/src/features/marketplace/components/MarketplaceDetailSkeleton.tsx
frontend/src/features/marketplace/components/MarketplaceDetailView.tsx
frontend/src/features/marketplace/components/MarketplaceLayout.tsx
frontend/src/features/marketplace/components/McpInstallButton.tsx
frontend/src/features/marketplace/components/McpMarketplaceCard.tsx
frontend/src/features/marketplace/components/McpMarketplaceDetailSheet.tsx
frontend/src/features/marketplace/components/McpMarketplaceDetailView.tsx
frontend/src/features/marketplace/i18n.ts
frontend/src/features/marketplace/screens/MarketplaceCliPage.tsx
frontend/src/features/marketplace/screens/MarketplaceMcpPage.tsx
frontend/src/features/marketplace/screens/MarketplacePage.tsx
frontend/src/features/mcp/components/McpFilterMenu.tsx
frontend/src/features/mcp/components/McpNeedsReviewServerList.tsx
frontend/src/features/mcp/components/McpNeedsReviewServerRow.tsx
frontend/src/features/mcp/components/McpServerCard.tsx
frontend/src/features/mcp/components/McpServerCardList.tsx
frontend/src/features/mcp/components/McpServerMatrixView.tsx
frontend/src/features/mcp/components/detail/McpMarketplaceLinkChip.tsx
frontend/src/features/mcp/components/detail/McpNeedsReviewDetailSheet.tsx
frontend/src/features/mcp/components/detail/McpNeedsReviewDetailView.tsx
frontend/src/features/mcp/components/detail/McpServerDetailSheet.tsx
frontend/src/features/mcp/components/detail/McpServerDetailView.tsx
frontend/src/features/mcp/components/edit/McpConfigChoiceDialog.tsx
frontend/src/features/mcp/i18n.ts
frontend/src/features/mcp/model/selectors.ts
frontend/src/features/mcp/screens/McpInUsePage.tsx
frontend/src/features/mcp/screens/McpNeedsReviewPage.tsx
frontend/src/features/overview/components/ExtensionPortfolio.tsx
frontend/src/features/overview/components/HarnessCoverageMap.tsx
frontend/src/features/overview/components/MarketplacePanel.tsx
frontend/src/features/overview/components/ReviewQueue.tsx
frontend/src/features/overview/components/StatisticsBand.tsx
frontend/src/features/overview/i18n.ts
frontend/src/features/overview/screens/OverviewPage.tsx
frontend/src/features/settings/components/SettingsHarnessCard.tsx
frontend/src/features/settings/i18n.ts
frontend/src/features/settings/model/use-settings-page-controller.ts
frontend/src/features/settings/screens/SettingsPage.tsx
frontend/src/features/settings/styles/settings.css
frontend/src/features/skills/components/cards/SkillInUseCard.tsx
frontend/src/features/skills/components/cards/SkillsInUseList.tsx
frontend/src/features/skills/components/cards/SkillsNeedsReviewList.tsx
frontend/src/features/skills/components/detail/SkillDetailContent.tsx
frontend/src/features/skills/components/detail/SkillDetailSkeleton.tsx
frontend/src/features/skills/components/detail/SkillDetailView.tsx
frontend/src/features/skills/components/dialogs/SkillActionConfirmDialog.tsx
frontend/src/features/skills/components/pane/SkillsEmptyState.tsx
frontend/src/features/skills/i18n.ts
frontend/src/features/skills/screens/SkillsInUsePage.tsx
frontend/src/features/skills/screens/SkillsNeedsReviewPage.tsx
frontend/src/features/skills/screens/SkillsWorkspacePage.tsx
frontend/src/features/slash-commands/components/SlashCommandFormDialog.tsx
frontend/src/features/slash-commands/components/SlashCommandList.tsx
frontend/src/features/slash-commands/components/detail/SlashCommandContentBlocks.tsx
frontend/src/features/slash-commands/components/detail/SlashCommandDetailView.tsx
frontend/src/features/slash-commands/components/detail/SlashCommandReviewDetailView.tsx
frontend/src/features/slash-commands/i18n.ts
frontend/src/features/slash-commands/screens/SlashCommandsPage.tsx
frontend/src/features/slash-commands/screens/SlashCommandsReviewPage.tsx
frontend/src/i18n/common.ts
frontend/src/i18n/glossary.ts
frontend/src/i18n/index.ts
frontend/src/i18n/locales.test.tsx
frontend/src/i18n/locales.tsx
frontend/src/styles/components/sidebar.css
frontend/src/test/render.tsx

Reviewer: @SI-RUI-ZHANG