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/+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);
     }