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/SplitMethodSelector.svelte

@@ -142,10 +142,11 @@
       <h3>Custom Split Amounts</h3>
       {#each users as user}
         <div class="split-input">
-          <label>{user}</label>
-          <input 
-            type="number" 
-            step="0.01" 
+          <label for="split_{user}">{user}</label>
+          <input
+            id="split_{user}"
+            type="number"
+            step="0.01"
             name="split_{user}"
             bind:value={splitAmounts[user]}
             placeholder="0.00"
@@ -161,10 +162,11 @@
       <p class="description">Enter personal amounts for each user. The remainder will be split equally.</p>
       {#each users as user}
         <div class="split-input">
-          <label>{user}</label>
-          <input 
-            type="number" 
-            step="0.01" 
+          <label for="personal_{user}">{user}</label>
+          <input
+            id="personal_{user}"
+            type="number"
+            step="0.01"
             min="0"
             name="personal_{user}"
             bind:value={personalAmounts[user]}