Replace __DITTO_CONFIG__ global with import.meta.env.DITTO_CONFIG and remove ThemeSchemaCompat

Move build-time ditto.json injection from a Vite define global to
import.meta.env.DITTO_CONFIG (a JSON string parsed and validated at
runtime via DittoConfigSchema). Remove the global type declaration
from vite-env.d.ts.

Drop ThemeSchemaCompat and its legacy "black"/"pink" migration code
from AppProvider and NostrSync — invalid theme values now simply fail
Zod validation.

Fix a latent bug where a partial feedSettings from ditto.json would
replace the full hardcoded defaults; defaultConfig now deep-merges
feedSettings.

vite.config.ts

@@ -136,7 +136,7 @@ export default defineConfig(() => {
     ...(publicDir ? [mergePublicDir(publicDir)] : []),
   ],
   define: {
-    __DITTO_CONFIG__: JSON.stringify(dittoConfig ?? null),
+    'import.meta.env.DITTO_CONFIG': JSON.stringify(JSON.stringify(dittoConfig ?? null)),
     'import.meta.env.VERSION': JSON.stringify(pkg.version),
     'import.meta.env.BUILD_DATE': JSON.stringify(new Date().toISOString()),
     'import.meta.env.COMMIT_SHA': JSON.stringify(getCommitSha()),