feat(web): add SearchInput header component (debounced URL sync)

Pre-fills from page.url.searchParams.get('q'); typing debounces 250ms
then goto('/search?q=…'); first nav from elsewhere pushes, subsequent
typing on /search replaces. Empty input on /search drops the param.
Escape clears the input. URL → value re-sync via \$effect uses
untrack() on the value comparison so it doesn't loop with typing.

web/src/lib/components/SearchInput.svelte

@@ -0,0 +1,57 @@
+<script lang="ts">
+  import { page } from '$app/state';
+  import { goto } from '$app/navigation';
+  import { untrack } from 'svelte';
+
+  let value = $state(page.url.searchParams.get('q') ?? '');
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+
+  // Re-sync local value when the URL changes from outside (back/forward,
+  // result-link clicks). Read `value` non-reactively to avoid a feedback
+  // loop where the user's typing would race against the URL re-read.
+  $effect(() => {
+    const fromUrl = page.url.searchParams.get('q') ?? '';
+    if (fromUrl !== untrack(() => value)) value = fromUrl;
+  });
+
+  function navigate(next: string) {
+    const onSearchPage = page.url.pathname.startsWith('/search');
+    const trimmed = next.trim();
+    if (trimmed === '') {
+      if (onSearchPage) goto('/search', { replaceState: true });
+      return;
+    }
+    goto(`/search?q=${encodeURIComponent(trimmed)}`, { replaceState: onSearchPage });
+  }
+
+  function onInput(e: Event) {
+    value = (e.currentTarget as HTMLInputElement).value;
+    if (timeout) clearTimeout(timeout);
+    timeout = setTimeout(() => {
+      timeout = null;
+      navigate(value);
+    }, 250);
+  }
+
+  function onKey(e: KeyboardEvent) {
+    if (e.key === 'Escape') {
+      e.preventDefault();
+      value = '';
+      if (timeout) {
+        clearTimeout(timeout);
+        timeout = null;
+      }
+      navigate('');
+    }
+  }
+</script>
+
+<input
+  type="search"
+  placeholder="Search artists, albums, tracks…"
+  aria-label="Search"
+  {value}
+  oninput={onInput}
+  onkeydown={onKey}
+  class="w-full max-w-md rounded border border-border bg-surface px-3 py-1 text-sm focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+/>

web/src/lib/components/SearchInput.test.ts

@@ -0,0 +1,94 @@
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/svelte';
+
+const state = vi.hoisted(() => ({
+  pageUrl: new URL('http://localhost/')
+}));
+
+vi.mock('$app/state', () => ({
+  page: { get url() { return state.pageUrl; } }
+}));
+
+vi.mock('$app/navigation', () => ({
+  goto: vi.fn()
+}));
+
+import SearchInput from './SearchInput.svelte';
+import { goto } from '$app/navigation';
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  state.pageUrl = new URL('http://localhost/');
+});
+
+afterEach(() => {
+  vi.clearAllMocks();
+  vi.useRealTimers();
+});
+
+describe('SearchInput', () => {
+  test('mount pre-fills value from page.url.searchParams', () => {
+    state.pageUrl = new URL('http://localhost/search?q=hello');
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    expect(input.value).toBe('hello');
+  });
+
+  test('typing fires goto once after 250ms debounce', async () => {
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: 'h' } });
+    expect(goto).not.toHaveBeenCalled();
+    vi.advanceTimersByTime(250);
+    expect(goto).toHaveBeenCalledTimes(1);
+    expect(goto).toHaveBeenCalledWith('/search?q=h', { replaceState: false });
+  });
+
+  test('rapid keystrokes debounce to a single goto', async () => {
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: 'h' } });
+    vi.advanceTimersByTime(100);
+    await fireEvent.input(input, { target: { value: 'he' } });
+    vi.advanceTimersByTime(100);
+    await fireEvent.input(input, { target: { value: 'hel' } });
+    vi.advanceTimersByTime(250);
+    expect(goto).toHaveBeenCalledTimes(1);
+    expect(goto).toHaveBeenCalledWith('/search?q=hel', { replaceState: false });
+  });
+
+  test('typing while on /search uses replaceState=true', async () => {
+    state.pageUrl = new URL('http://localhost/search?q=h');
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: 'he' } });
+    vi.advanceTimersByTime(250);
+    expect(goto).toHaveBeenCalledWith('/search?q=he', { replaceState: true });
+  });
+
+  test('clearing input on /search calls goto("/search") (drops ?q=)', async () => {
+    state.pageUrl = new URL('http://localhost/search?q=h');
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: '' } });
+    vi.advanceTimersByTime(250);
+    expect(goto).toHaveBeenCalledWith('/search', { replaceState: true });
+  });
+
+  test('Escape clears the input value', async () => {
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: 'hello' } });
+    expect(input.value).toBe('hello');
+    await fireEvent.keyDown(input, { key: 'Escape' });
+    expect(input.value).toBe('');
+  });
+
+  test('encodeURIComponent applied to special chars', async () => {
+    render(SearchInput);
+    const input = screen.getByRole('searchbox') as HTMLInputElement;
+    await fireEvent.input(input, { target: { value: 'a b&c' } });
+    vi.advanceTimersByTime(250);
+    expect(goto).toHaveBeenCalledWith('/search?q=a%20b%26c', { replaceState: false });
+  });
+});