feat: implement email validation, CI/CD pipeline, migration history, and web templates

Email validation:
- Replace @/. check with net/mail.ParseAddress on register
- Add size limit check (max 254 chars, RFC 5321)

CI/CD Pipeline:
- Add .gitea/workflows/ci.yml (lint → test → build → docker push)
- Registry: gitea.mrixs.me/Mrixs/MrixsCraft-server
- Push only on main branch

Database:
- Add migrations/002_migration_history.sql (tracking applied migrations)
- Add migrations/README.md (manual apply instructions)

Web Templates:
- Add base.html with Minecraft-themed layout (dark + green accent)
- Add index.html, login.html, register.html with POST forms
- Rewrite templates.go for data-driven rendering with pageData struct
- Fallback placeholder preserved when templates dir missing

migrations/README.md

@@ -0,0 +1,14 @@
+# Migrations
+
+Applied manually in order:
+
+```bash
+psql "$DATABASE_URL" -f migrations/001_init.sql        # schema
+psql "$DATABASE_URL" -f migrations/002_migration_history.sql  # tracking
+```
+
+Check applied:
+
+```sql
+SELECT * FROM migration_history ORDER BY id;
+```