refactor: $app/stores → $app/state, legacy stores → runes

Codemod-driven migration of 55 .svelte files from the deprecated
$app/stores module to the rune-based $app/state ($page.x → page.x,
no auto-subscription wrapper). Two custom writable() stores converted
to .svelte.ts factory functions matching the existing theme store
pattern, with consumers updated to use .value getters and the explicit
.set() method.

UserHeader.svelte's login link now guards page.url.search behind
the browser flag — search-param access throws during prerender, and
this defensive change unblocks future prerender adoption on any page
that includes the header.

scripts/codemod-app-stores-to-state.ts

@@ -0,0 +1,84 @@
+/**
+ * Migrate `$app/stores` (deprecated) to `$app/state` (rune-based).
+ *
+ * For each .svelte file:
+ *   - Rewrite `from '$app/stores'` → `from '$app/state'`
+ *   - For each named import, drop the `$` prefix from auto-subscriptions:
+ *       `$page.url.pathname` → `page.url.pathname`
+ *       `$navigating` → `navigating`
+ *       `$updated` → `updated`
+ *     Aliased imports (`page as appPage`) are tracked, so `$appPage` becomes `appPage`.
+ *
+ * Skips:
+ *   - Non-.svelte files (server-only code uses getRequestEvent instead).
+ *   - Files importing other things from $app/stores that don't have a state equivalent
+ *     (none observed in this repo).
+ *
+ * Run:  pnpm exec vite-node scripts/codemod-app-stores-to-state.ts [--dry]
+ */
+
+import { readFileSync, writeFileSync, readdirSync, statSync } from 'node:fs';
+import { join, extname } from 'node:path';
+
+const SRC = 'src';
+const DRY = process.argv.includes('--dry');
+
+const STORES_IMPORT_RE =
+	/import\s*\{([^}]+)\}\s*from\s*['"]\$app\/stores['"]\s*;?/;
+
+function walk(dir: string, out: string[] = []): string[] {
+	for (const name of readdirSync(dir)) {
+		const p = join(dir, name);
+		const s = statSync(p);
+		if (s.isDirectory()) walk(p, out);
+		else if (extname(p) === '.svelte') out.push(p);
+	}
+	return out;
+}
+
+function parseImports(inner: string): Array<{ orig: string; local: string }> {
+	return inner
+		.split(',')
+		.map((s) => s.trim())
+		.filter(Boolean)
+		.map((spec) => {
+			const m = spec.match(/^(\w+)(?:\s+as\s+(\w+))?$/);
+			if (!m) return null;
+			return { orig: m[1], local: m[2] ?? m[1] };
+		})
+		.filter((x): x is { orig: string; local: string } => x !== null);
+}
+
+function rewriteFile(src: string): { code: string; changed: boolean } {
+	const m = STORES_IMPORT_RE.exec(src);
+	if (!m) return { code: src, changed: false };
+
+	const imports = parseImports(m[1]);
+	if (imports.length === 0) return { code: src, changed: false };
+
+	// Replace the import path; preserve the same import shape.
+	let out = src.replace(STORES_IMPORT_RE, (full) =>
+		full.replace(/['"]\$app\/stores['"]/, "'$app/state'")
+	);
+
+	// Drop `$` prefix from each local name where it appears as a store
+	// auto-subscription (i.e. $name followed by a non-word boundary).
+	for (const { local } of imports) {
+		const re = new RegExp(`\\$${local}\\b`, 'g');
+		out = out.replace(re, local);
+	}
+
+	return { code: out, changed: out !== src };
+}
+
+const files = walk(SRC);
+let changed = 0;
+for (const f of files) {
+	const orig = readFileSync(f, 'utf8');
+	const { code, changed: didChange } = rewriteFile(orig);
+	if (!didChange) continue;
+	if (!DRY) writeFileSync(f, code);
+	changed++;
+	console.log(`  ${f}`);
+}
+console.log(`\n${DRY ? '[dry] ' : ''}${changed} files migrated`);