Support build-time ditto.json config file

Read an optional ditto.json (or CONFIG_FILE env) at build time, validate
it with Zod, and inject the result via Vite's define. At runtime the
build-time values are merged between the hardcoded defaults and user
localStorage, giving precedence: user > build-time > hardcoded.

Extract canonical Zod schemas into config/schema.ts (no path aliases)
so they can be imported from both vite.config.ts and runtime code.
Move AppConfigSchema out of AppProvider into src/lib/schemas.ts to
eliminate duplication.

vite.config.ts

@@ -1,8 +1,34 @@
+import fs from "node:fs";
 import path from "node:path";
 
 import react from "@vitejs/plugin-react-swc";
 import { defineConfig } from "vitest/config";
 
+import { DittoConfigSchema } from "./config/schema";
+
+/**
+ * Load and validate the build-time ditto.json configuration file.
+ * Returns the parsed config object, or `undefined` if the file doesn't exist.
+ * Set the CONFIG_FILE env var to override the default path ("./ditto.json").
+ */
+function loadDittoConfig(): object | undefined {
+  const configPath = path.resolve(process.env.CONFIG_FILE ?? "./ditto.json");
+
+  let raw: string;
+  try {
+    raw = fs.readFileSync(configPath, "utf-8");
+  } catch {
+    // File not found — no build-time config
+    return undefined;
+  }
+
+  const json = JSON.parse(raw);
+  const result = DittoConfigSchema.parse(json);
+  return result;
+}
+
+const dittoConfig = loadDittoConfig();
+
 // https://vitejs.dev/config/
 export default defineConfig(() => ({
   server: {
@@ -12,6 +38,9 @@ export default defineConfig(() => ({
   plugins: [
     react(),
   ],
+  define: {
+    __DITTO_CONFIG__: JSON.stringify(dittoConfig ?? null),
+  },
   test: {
     globals: true,
     environment: 'jsdom',