feat(ai): implement agent-redesign plan with enhanced AI travel features

Phase 1 - Configuration Infrastructure (WS1):
- Add instance-level AI env vars (VOYAGE_AI_PROVIDER, VOYAGE_AI_MODEL, VOYAGE_AI_API_KEY)
- Implement fallback chain: user key → instance key → error
- Add UserAISettings model for per-user provider/model preferences
- Enhance provider catalog with instance_configured and user_configured flags
- Optimize provider catalog to avoid N+1 queries

Phase 1 - User Preference Learning (WS2):
- Add Travel Preferences tab to Settings page
- Improve preference formatting in system prompt with emoji headers
- Add multi-user preference aggregation for shared collections

Phase 2 - Day-Level Suggestions Modal (WS3):
- Create ItinerarySuggestionModal with 3-step flow (category → filters → results)
- Add AI suggestions button to itinerary Add dropdown
- Support restaurant, activity, event, and lodging categories
- Backend endpoint POST /api/chat/suggestions/day/ with context-aware prompts

Phase 3 - Collection-Level Chat Improvements (WS4):
- Inject collection context (destination, dates) into chat system prompt
- Add quick action buttons for common queries
- Add 'Add to itinerary' button on search_places results
- Update chat UI with travel-themed branding and improved tool result cards

Phase 3 - Web Search Capability (WS5):
- Add web_search agent tool using DuckDuckGo
- Support location_context parameter for biased results
- Handle rate limiting gracefully

Phase 4 - Extensibility Architecture (WS6):
- Implement decorator-based @agent_tool registry
- Convert existing tools to use decorators
- Add GET /api/chat/capabilities/ endpoint for tool discovery
- Refactor execute_tool() to use registry pattern

frontend/src/routes/collections/[id]/+page.svelte

@@ -256,6 +256,29 @@
 	// Enforce recommendations visibility only for owner/shared users
 	$: availableViews.recommendations = !!canModifyCollection;
 
+	function deriveCollectionDestination(current: Collection | null): string | undefined {
+		if (!current?.locations?.length) {
+			return undefined;
+		}
+
+		const firstLocation = current.locations.find((loc) =>
+			Boolean(loc.city?.name || loc.country?.name || loc.location || loc.name)
+		);
+		if (!firstLocation) {
+			return undefined;
+		}
+
+		const cityName = firstLocation.city?.name;
+		const countryName = firstLocation.country?.name;
+		if (cityName && countryName) {
+			return `${cityName}, ${countryName}`;
+		}
+
+		return cityName || countryName || firstLocation.location || firstLocation.name || undefined;
+	}
+
+	$: collectionDestination = deriveCollectionDestination(collection);
+
 	// Build calendar events from collection visits
 	type TimezoneMode = 'event' | 'local';
 
@@ -1261,7 +1284,14 @@
 				<!-- Recommendations View -->
 				{#if currentView === 'recommendations'}
 					<div class="space-y-8">
-						<AITravelChat embedded={true} />
+						<AITravelChat
+							embedded={true}
+							collectionId={collection.id}
+							collectionName={collection.name}
+							startDate={collection.start_date || undefined}
+							endDate={collection.end_date || undefined}
+							destination={collectionDestination}
+						/>
 						<CollectionRecommendationView bind:collection user={data.user} />
 					</div>
 				{/if}