feat: complete Svelte 5 migration across entire application

Migrated all components and routes from Svelte 4 to Svelte 5 syntax:

- Converted export let → $props() with generic type syntax
- Replaced createEventDispatcher → callback props
- Migrated $: reactive statements → $derived() and $effect()
- Updated two-way bindings with $bindable()
- Fixed TypeScript syntax: added lang="ts" to script tags
- Converted inline type annotations to generic parameter syntax

- Updated deprecated event directives to Svelte 5 syntax:
  - on:click → onclick
  - on:submit → onsubmit
  - on:change → onchange

- Converted deprecated <slot> elements → {@render children()}
- Updated slot props to Snippet types
- Fixed season/icon selector components with {#snippet} blocks

- Fixed non-reactive state by converting let → $state()
- Fixed infinite loop in EnhancedBalance by converting $effect → $derived
- Fixed Chart.js integration by converting $state proxies to plain arrays
- Updated cospend dashboard and payment pages with proper reactivity

- Migrated 20+ route files from export let data → $props()
- Fixed TypeScript type annotations in page components
- Updated reactive statements in error and cospend routes

- Removed invalid onchange attribute from Toggle component
- Fixed modal ID isolation in CreateIngredientList/CreateStepList
- Fixed dark mode button visibility in TranslationApproval
- Build now succeeds with zero deprecation warnings

All functionality tested and working. No breaking changes to user experience.

src/lib/components/ProfilePicture.svelte

@@ -1,12 +1,10 @@
-<script>
-  export let username;
-  export let size = 40; // Default size in pixels
-  export let alt = '';
+<script lang="ts">
+  let { username, size = 40, alt = '' } = $props<{ username: string, size?: number, alt?: string }>();
 
-  let imageError = false;
+  let imageError = $state(false);
 
-  $: profileUrl = `https://bocken.org/static/user/full/${username}.webp`;
-  $: altText = alt || `${username}'s profile picture`;
+  let profileUrl = $derived(`https://bocken.org/static/user/full/${username}.webp`);
+  let altText = $derived(alt || `${username}'s profile picture`);
 
   function handleError() {
     imageError = true;
@@ -27,7 +25,7 @@
     <img
       src={profileUrl}
       alt={altText}
-      on:error={handleError}
+      onerror={handleError}
       loading="lazy"
     />
   {:else}