refactor: complete Svelte 5 migration to eliminate all deprecation warnings

Migrated all components and routes to Svelte 5 syntax standards:

Event Handlers:
- Updated all deprecated on:* directives to new on* attribute syntax
- Changed on:click → onclick, on:keydown → onkeydown, on:input → oninput
- Updated on:blur, on:focus, on:load, on:submit, on:cancel handlers

Reactive State:
- Added $state() declarations for all reactive variables
- Fixed non-reactive update warnings in layout and component files

Component API:
- Replaced <slot /> with {@render children()} pattern
- Added children prop to components using slots

Accessibility:
- Added id attributes to inputs and for attributes to labels
- Fixed label-control associations across forms
- Removed event listeners from non-interactive elements

HTML Fixes:
- Fixed self-closing textarea tags
- Corrected implicitly closed elements
- Proper element nesting

CSS Cleanup:
- Removed 20+ unused CSS selectors across components
- Cleaned up orphaned styles from refactoring

All vite-plugin-svelte warnings resolved. Codebase now fully compliant with Svelte 5.

src/routes/cospend/+layout.svelte

@@ -9,14 +9,10 @@
   import UserHeader from '$lib/components/UserHeader.svelte';
 
   let { data, children } = $props();
-  
-  let showModal = false;
-  let paymentId = null;
-  let user;
-  
-  if (data.session) {
-    user = data.session.user;
-  }
+
+  let showModal = $state(false);
+  let paymentId = $state(null);
+  let user = $state(data.session?.user);
 
   $effect(() => {
     // Check if URL contains payment view route OR if we have paymentId in state

src/routes/cospend/+page.svelte

@@ -317,10 +317,6 @@
     color: var(--red);
   }
 
-  .even {
-    color: var(--nord3);
-  }
-
   .actions {
     display: flex;
     gap: 1rem;
@@ -617,16 +613,6 @@
     }
   }
 
-  .you-badge {
-    background-color: var(--blue);
-    color: white;
-    padding: 0.125rem 0.5rem;
-    border-radius: 1rem;
-    font-size: 0.75rem;
-    font-weight: 500;
-    display: inline-block;
-  }
-
   .activity-amount {
     font-weight: bold;
     font-size: 1rem;
@@ -645,7 +631,7 @@
     flex-wrap: wrap;
   }
 
-  .paid-by, .payment-date {
+  .payment-date {
     color: var(--nord3);
     font-size: 0.9rem;
   }
@@ -659,7 +645,7 @@
   }
 
   @media (prefers-color-scheme: dark) {
-    .paid-by, .payment-date {
+    .payment-date {
       color: var(--nord4);
     }
 

src/routes/cospend/payments/add/+page.svelte

@@ -803,18 +803,10 @@
     display: block;
   }
 
-  .js-enhanced {
-    display: none;
-  }
-
   :global(body.js-loaded) .no-js-only {
     display: none;
   }
 
-  :global(body.js-loaded) .js-enhanced {
-    display: block;
-  }
-
   .manual-users textarea {
     width: 100%;
     padding: 0.75rem;

src/routes/cospend/payments/edit/[id]/+page.svelte

@@ -463,32 +463,6 @@
     gap: 2rem;
   }
 
-  .form-section {
-    background: var(--nord6);
-    padding: 1.5rem;
-    border-radius: 0.75rem;
-    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
-    border: 1px solid var(--nord4);
-  }
-
-  .form-section h2 {
-    margin-top: 0;
-    margin-bottom: 1rem;
-    color: var(--nord0);
-    font-size: 1.25rem;
-  }
-
-  @media (prefers-color-scheme: dark) {
-    .form-section {
-      background: var(--nord1);
-      border-color: var(--nord2);
-    }
-
-    .form-section h2 {
-      color: var(--font-default-dark);
-    }
-  }
-
   .form-group {
     margin-bottom: 1rem;
   }

src/routes/cospend/payments/view/[id]/+page.svelte

@@ -479,15 +479,6 @@
       margin-left: 0;
     }
 
-    .header-content {
-      flex-direction: column;
-      align-items: stretch;
-    }
-
-    .header-actions {
-      justify-content: center;
-    }
-
     .info-grid {
       grid-template-columns: 1fr;
     }