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

internal/api/api.go

@@ -7,6 +7,7 @@ import (
 	"image/png"
 	"io"
 	"net/http"
+	"net/mail"
 	"os"
 	"path/filepath"
 	"strings"
@@ -156,8 +157,12 @@ func (h *Handler) register(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Basic email validation.
-	if !strings.Contains(req.Email, "@") || !strings.Contains(req.Email, ".") {
+	// Basic email validation (RFC 5321).
+	if len(req.Email) > 254 {
+		utils.WriteError(w, http.StatusBadRequest, "Email too long (max 254 characters)")
+		return
+	}
+	if _, err := mail.ParseAddress(req.Email); err != nil {
 		utils.WriteError(w, http.StatusBadRequest, "Invalid email address")
 		return
 	}