fix: resolve all 1008 svelte-check type errors across codebase

Add type annotations, JSDoc types, null checks, and proper generics
to eliminate all svelte-check errors. Key changes include:
- Type $state(null) variables to avoid 'never' inference
- Add JSDoc typedefs for plain <script> components
- Fix mongoose model typing with Model<any> to avoid union complexity
- Add App.Error/App.PageState interfaces in app.d.ts
- Fix tuple types to array types in types.ts
- Type catch block errors and API handler params
- Add null safety for DOM queries and optional chaining
- Add standard line-clamp property alongside -webkit- prefix

src/routes/api/cospend/payments/+server.ts

@@ -102,7 +102,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
       exchangeRate = conversion.exchangeRate;
     } catch (e) {
       console.error('Currency conversion error:', e);
-      throw error(400, `Failed to convert ${inputCurrency} to CHF: ${e.message}`);
+      throw error(400, `Failed to convert ${inputCurrency} to CHF: ${e instanceof Error ? e.message : String(e)}`);
     }
   }
 
@@ -146,7 +146,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
       };
     });
 
-    const splitPromises = convertedSplits.map((split) => {
+    const splitPromises = convertedSplits.map((split: any) => {
       return PaymentSplit.create(split);
     });