feat: add /bookshelf route and update redirects

- Add /bookshelf route as default page for logged-in users
- Update login and register handlers to redirect to /bookshelf
- Update homepage to auto-redirect to /bookshelf when logged in
- Preserve /dashboard route for backward compatibility
- Update test redirects to use /bookshelf

Changes:
- main.go: Add /bookshelf protected route
- auth.go: Change login/register redirects from /api/dashboard to /bookshelf (2 locations)
- edge_cases_test.go: Update test redirect to /bookshelf
- Maintains backward compatibility with existing /dashboard route

This makes the beautiful bookshelf the default landing page
for all authenticated users while keeping the old dashboard accessible.

cmd/server/main.go

@@ -168,7 +168,29 @@ func main() {
 		}
 	}()
 
-	// Dashboard route (protected)
+	// Bookshelf route (protected) - new default for logged-in users
+	protected.GET("/bookshelf", func(c echo.Context) error {
+		userID := c.Get("user_id").(string)
+		userEmail := c.Get("user_email").(string)
+		userUsername := c.Get("user_username").(string)
+		userRole := c.Get("user_role").(string)
+
+		user := templates.User{
+			ID:       userID,
+			Email:    userEmail,
+			Username: userUsername,
+			Role:     userRole,
+		}
+
+		var buf bytes.Buffer
+		err := templates.BookShelf(user).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
+	// Dashboard route (protected) - keep for backward compatibility
 	protected.GET("/dashboard", func(c echo.Context) error {
 		userID := c.Get("user_id").(string)
 		userEmail := c.Get("user_email").(string)