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/templates/html/register.html

@@ -0,0 +1,43 @@
+{{define "content"}}
+<div class="card">
+    <h1>Регистрация</h1>
+    <form id="regForm">
+        <label for="username">Никнейм</label>
+        <input type="text" id="username" name="username" required minlength="3" maxlength="20" autocomplete="username">
+
+        <label for="email">Email</label>
+        <input type="email" id="email" name="email" required autocomplete="email">
+
+        <label for="password">Пароль</label>
+        <input type="password" id="password" name="password" required minlength="6" autocomplete="new-password">
+
+        <button type="submit" class="btn">Создать аккаунт</button>
+    </form>
+    <p id="error" style="color: var(--error); margin-top: 0.75rem; display: none;"></p>
+    <p style="margin-top: 1rem;">Уже есть аккаунт? <a href="/login">Войти</a></p>
+</div>
+
+<script>
+document.getElementById('regForm').addEventListener('submit', async (e) => {
+    e.preventDefault();
+    const form = new FormData(e.target);
+    const res = await fetch('/api/web/register', {
+        method: 'POST',
+        headers: {'Content-Type': 'application/json'},
+        body: JSON.stringify({
+            username: form.get('username'),
+            email: form.get('email'),
+            password: form.get('password')
+        })
+    });
+    const data = await res.json();
+    if (!res.ok) {
+        document.getElementById('error').textContent = data.error || 'Ошибка регистрации';
+        document.getElementById('error').style.display = 'block';
+        return;
+    }
+    // Registration successful — go to login.
+    window.location.href = '/login';
+});
+</script>
+{{end}}