feat(dev): proxy /pb to PocketBase in ng serve

Add proxy.conf.mjs mirroring `location /pb/` from nginx: the prefix is
stripped, target comes from PB_URL (empty value falls back to the default,
same as the SSR provider). Cover the config with tests in tests/lint and
update docs.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tsconfig.tests.json

@@ -5,6 +5,7 @@
         "strict": true,
         "noEmit": true,
         "allowImportingTsExtensions": true,
+        "allowJs": true,
         "types": ["node"]
     },
     "files": [],
@@ -12,6 +13,7 @@
         "tests/**/*.ts",
         "vitest.api.config.mts",
         "vitest.integration.config.mts",
-        "vitest.lint.config.mts"
+        "vitest.lint.config.mts",
+        "proxy.conf.mjs"
     ]
 }