feat: add admin publishing workflow and yar theme

Add Go/Postgres admin APIs, Angular admin UI, manual build flow, asset uploads, markdown import/export, configurable slug generation, and the Yar reading theme. Exclude local docs and generated development artifacts from version control.

frontend/admin/README.md

@@ -0,0 +1,30 @@
+# Osaet Admin
+
+Angular admin UI for the Osaet Gin API.
+
+## Development
+
+Start the Go admin API first:
+
+```bash
+cd /home/yarnom/Codes/osaet/backend
+DATABASE_URL='postgres://yarnom:数据库密码@10.66.0.30:5432/osaet?sslmode=disable' \
+go run ./cmd/osaet-admin serve
+```
+
+Install dependencies and start Angular:
+
+```bash
+cd /home/yarnom/Codes/osaet/frontend/admin
+npm install
+npm start
+```
+
+Open:
+
+```text
+http://127.0.0.1:4200/
+```
+
+The Angular dev server proxies `/api`, `/healthz`, and `/readyz` to
+`http://127.0.0.1:8080`.