Migrate to Svelte 5 Runes syntax

[Copilot]

This PR completes the migration from Svelte 4 patterns to Svelte 5 Runes syntax while maintaining full functionality and backward compatibility.

Changes Made

Reactive State Migration

• Contact.svelte: Converted form variables from regular let declarations to $state() for proper reactivity
• Contact.svelte: Replaced $: reactive statement with $derived() for computed triggerContent
• Hero.svelte: Replaced onMount() + store subscription pattern with $derived() using direct $mode access

Event Handler Modernization

• Contact.svelte: Updated on:submit to onsubmit
• Navbar.svelte: Updated on:click handlers to onclick

Before/After Examples

Contact Form (Before):

let contactForm: ContactFormProps = { /* ... */ };
let invalidInputForm = false;
$: triggerContent = subjects.find(s => s.value === contactForm.subject)?.label ?? "Select a subject";

Contact Form (After):

let contactForm = $state<ContactFormProps>({ /* ... */ });
let invalidInputForm = $state(false);
const triggerContent = $derived(subjects.find(s => s.value === contactForm.subject)?.label ?? "Select a subject");

Hero Component (Before):

let imageSrc = 'hero-image-light.jpg';
onMount(() => {
    mode.subscribe(currentMode => {
        imageSrc = currentMode === 'dark' ? 'hero-image-dark.jpg' : 'hero-image-light.jpg';
    });
});

Hero Component (After):

const imageSrc = $derived($mode === 'dark' ? 'hero-image-dark.jpg' : 'hero-image-light.jpg');

Testing

• ✅ Project builds successfully with no errors
• ✅ All interactive features tested (dark mode toggle, contact form, carousel)
• ✅ Form reactivity verified (dropdown updates trigger computed values)
• ✅ No breaking changes to user experience
• ✅ Build output remains consistent

The migration adopts modern Svelte 5 patterns while keeping changes minimal and surgical. All existing functionality is preserved, and the app now fully leverages Svelte 5's new reactivity system.

Fixes #1.

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