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>

angular.json

@@ -59,6 +59,9 @@
                     "defaultConfiguration": "production"
                 },
                 "serve": {
+                    "options": {
+                        "proxyConfig": "proxy.conf.mjs"
+                    },
                     "builder": "@angular/build:dev-server",
                     "configurations": {
                         "production": {