feat: unified error handling + AI service modularisation

Error handling (Prompt 2):
- src/services/errors.ts: AppError class, normalizeError(), throwServiceError() helper
- 6 services wrapped with try/catch (property, match, need, shortlist, futureSignal, inquiry)
- inquiryService aligned from custom ServiceResult<T> to standard ServiceResponse types
- Results, MatchCenter, FutureAvailability pages show <ErrorState onRetry> on query failure

AI modularisation (Prompt 3):
- src/services/aiService.ts reduced from 755 → 19 lines (barrel re-export)
- src/services/ai/IAIService.ts: typed interface + all response types
- src/services/ai/mock/: needParser, compareBuilder, decisionBrief, listingParser, MockAIService
- src/services/ai/openrouter/OpenRouterAIService.ts: model-agnostic skeleton
- src/services/ai/prompts/: 4 prompt template files (needParsing, matchExplanation, compareSummary, decisionBrief)
- src/services/ai/index.ts: factory selects Mock or OpenRouter via VITE_USE_REAL_AI flag
- All existing import paths unchanged — zero call-site modifications

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

src/services/ai/index.ts

@@ -0,0 +1,20 @@
+/**
+ * AI Service Factory
+ *
+ * Selects the active implementation based on the VITE_USE_REAL_AI feature flag.
+ *
+ * Mock mode (default):  no API key required, all responses are deterministic.
+ * Real mode:            set VITE_USE_REAL_AI=true + VITE_OPENROUTER_API_KEY in .env
+ */
+import { MockAIService } from './mock/MockAIService'
+import { OpenRouterAIService } from './openrouter/OpenRouterAIService'
+import type { IAIService } from './IAIService'
+
+const useRealAI = import.meta.env.VITE_USE_REAL_AI === 'true'
+  && !!import.meta.env.VITE_OPENROUTER_API_KEY
+
+export const aiService: IAIService = useRealAI ? OpenRouterAIService : MockAIService
+
+// Named export so callers can reach the concrete impl when needed
+export { MockAIService, OpenRouterAIService }
+export type { IAIService }