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.

.gitignore

@@ -33,6 +33,9 @@ deploy.sh
 .env.*
 !.env.example
 
+# Build-time configuration
+ditto.json
+
 # Android build outputs and sensitive files
 *.aab
 resources/