feat: Schattenmarkt UX — separate section, relevance bridge, source links

Signals are now in their own feed section with an explanation banner
so users understand they are not confirmed properties. Each signal card
shows WHY it appears in their search (e.g. 'Fläche könnte frei werden').
Source pills are now clickable links. FutureAvailabilityContextPanel
redesigned with full AI summary, source button, and relevance bridge.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/features/matching/matchCardAdapter.ts

@@ -113,6 +113,7 @@ export function buildMatchCardViewModel(
     isStaleData: false,
     // Schattenmarkt signal fields
     signalSourceType: signal?.source?.type,
+    signalSourceUrl: signal?.source?.url,
     signalSourceCredibility: signal?.source?.credibility,
     signalProbability: signal?.probability,
     signalMarketIndicator: signal?.marketIndicator,