feat: social monitoring UI overview page and project integration

[arberx]

Summary

Adds social monitoring pages and components to the web dashboard, including a dedicated Social overview page (/social), a Social Signals section in the Project Command Center, a Social sparkline column in the portfolio projects table, and a Social Platforms section in Settings.

Changes

• api.ts: Added social API types (ApiSocialPlatform, ApiSocialMention, ApiSocialSummary) and stub fetch functions (fetchSocialSummary, fetchProjectSocialMentions, connectSocialPlatform, disconnectSocialPlatform)
• view-models.ts: Added social view model types (SocialPlatformVm, SocialMentionVm, SocialOverviewVm, ProjectSocialSignalsVm); added optional socialTrend?: number[] to ProjectCommandCenterVm
• src/pages/SocialPage.tsx (new): Social overview page with hero ScoreGauge metrics row (Total Mentions, Sentiment Score, Domain Links), platform breakdown table, and paginated/filterable recent mentions table with ToneBadge sentiment
• src/components/project/SocialSignalsSection.tsx (new): Self-contained Social Signals section for the Project Command Center — shows mention count, sentiment split, top hashtag/subreddit, 7d sparkline trend, platform filter chips, and a social mentions table
• src/pages/ProjectPage.tsx: Import and render SocialSignalsSection below visibility evidence, before competitor table
• src/pages/ProjectsPage.tsx: Added Sparkline import and Social (7d) column to portfolio projects table
• src/pages/SettingsPage.tsx: Added SocialPlatformCard component and Social Platforms section (Twitter/X, Reddit, LinkedIn) with API key input, connect/disconnect buttons
• src/router/routes.tsx: Added /social route pointing to SocialPage
• src/App.tsx: Added MessageSquare icon import, Social nav link in sidebar and mobile nav, Social breadcrumb label

Testing

All 507 tests pass (pnpm typecheck && pnpm lint && pnpm test). TypeScript compiles cleanly with no errors. Social pages gracefully handle missing backend (shows informative empty states since Social API endpoints are not yet implemented — tracked in the dependent issue).

Fixes #101