fix(chat): improve OpenCode Zen integration and error handling

- Fetch models dynamically from OpenCode Zen API (36 models vs 5 hardcoded)
- Add function calling support check before using tools
- Add retry logic (num_retries=2) for transient failures
- Improve logging for debugging API calls and errors
- Update system prompt for multi-stop itinerary context
- Clean up unused imports in frontend components
- Remove deleted views.py (moved to views/__init__.py)

frontend/src/lib/components/DateTimeInput.svelte

@@ -49,8 +49,15 @@
 		disabled={disabled || readonly}
 	>
 		<span class={displayDateTime ? '' : 'opacity-40'}>{displayDateTime || 'DD/MM/YYYY HH:MM'}</span>
-		<svg xmlns="http://www.w3.org/2000/svg" class="h-4 w-4 opacity-60" viewBox="0 0 24 24" fill="currentColor">
-			<path d="M19 3h-1V1h-2v2H8V1H6v2H5c-1.1 0-2 .9-2 2v16c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2zm0 18H5V8h14v13zM7 10h5v5H7z"/>
+		<svg
+			xmlns="http://www.w3.org/2000/svg"
+			class="h-4 w-4 opacity-60"
+			viewBox="0 0 24 24"
+			fill="currentColor"
+		>
+			<path
+				d="M19 3h-1V1h-2v2H8V1H6v2H5c-1.1 0-2 .9-2 2v16c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2zm0 18H5V8h14v13zM7 10h5v5H7z"
+			/>
 		</svg>
 	</button>
 	<input