feat: v4.0.1 — onboarding, landing page, smart redirect, register GET, 404 stylé, API unifiée

- Landing page / pour visiteurs non-auth (template landing.html)
- Redirection / intelligente: non-auth → landing, auth sans Gitea → local workspace, auth+Gitea → dashboard
- GET /auth/register — page d'inscription dédiée (tab register actif)
- 404 handler stylisé thème Notion sombre (JSON pour /api/*, HTML pour le reste)
- Alias /api/pages GET/POST → /board/api/pages (307 redirect)
- 133 tests passent

app/routers/auth.py

@@ -85,6 +85,27 @@ async function handleLogin(e){e.preventDefault();const email=document.getElement
 </html>"""
 
 
+@router.get("/register")
+async def register_page(request: Request):
+    """Show the registration page (local login page with register tab active)."""
+    return HTMLResponse(LOCAL_LOGIN_HTML.replace(
+        'class="tab active" onclick="switchTab(\'login\')"',
+        'class="tab" onclick="switchTab(\'login\')"'
+    ).replace(
+        'class="tab" onclick="switchTab(\'register\')"',
+        'class="tab active" onclick="switchTab(\'register\')"'
+    ).replace(
+        'let mode=\'login\';',
+        'let mode=\'register\';'
+    ).replace(
+        'id="name-group" style="display:none"',
+        'id="name-group" style="display:block"'
+    ).replace(
+        'id="submit-btn">Login<',
+        'id="submit-btn">Register<'
+    ), status_code=200)
+
+
 @router.get("/login")
 async def login(request: Request, provider: str = Query("gitea")):
     """Redirect to OAuth2 authorize page or show local login page."""