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/lib/components/PaymentModal.svelte

@@ -462,7 +462,7 @@
     flex-shrink: 0;
   }
 
-  .btn-primary, .btn-secondary, .btn-danger {
+  .btn-secondary {
     padding: 0.75rem 1.5rem;
     border-radius: 0.5rem;
     font-size: 1rem;
@@ -472,20 +472,6 @@
     text-decoration: none;
     display: inline-block;
     text-align: center;
-  }
-
-  .btn-primary {
-    background-color: var(--blue);
-    color: white;
-  }
-
-  .btn-primary:hover:not(:disabled) {
-    background-color: var(--nord10);
-    transform: translateY(-1px);
-    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.15);
-  }
-
-  .btn-secondary {
     background-color: var(--nord5);
     color: var(--nord0);
     border: 1px solid var(--nord4);
@@ -496,21 +482,6 @@
     transform: translateY(-1px);
   }
 
-  .btn-danger {
-    background-color: var(--red);
-    color: white;
-  }
-
-  .btn-danger:hover:not(:disabled) {
-    background-color: var(--nord11);
-    transform: translateY(-1px);
-  }
-
-  .btn-danger:disabled {
-    opacity: 0.6;
-    cursor: not-allowed;
-  }
-
   @media (prefers-color-scheme: dark) {
     .panel-content {
       background: var(--nord1);