feat(router): improve error handling with dedicated error pages

- Add renderErrorPage helper for consistent error rendering
- Add ensureUserExistsMiddleware to detect deleted users and redirect to login
- Add catch-all 404 handler for unknown routes
- Gracefully handle data loading failures with error messages instead of crashing
- Log errors for debugging while still rendering pages

internal/router/router.go

@@ -8,6 +8,8 @@ import (
 	ratelimit "bookhoard/internal/middleware"
 	"bookhoard/internal/services"
 	"bookhoard/internal/sync"
+	"bookhoard/templates"
+	"bytes"
 	"log"
 	"net/http"
 	"strings"
@@ -105,6 +107,45 @@ func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 	})
 }
 
+// ensureUserExistsMiddleware checks if the authenticated user still exists in the database
+func ensureUserExistsMiddleware(cfg *Config) echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			userIDStr, ok := c.Get("user_id").(string)
+			if !ok {
+				return next(c)
+			}
+
+			userUUID, err := uuid.Parse(userIDStr)
+			if err != nil {
+				log.Printf("Invalid UUID in user existence check: %v", err)
+				return next(c)
+			}
+
+			// Check if user exists in database
+			_, err = cfg.Queries.GetUser(c.Request().Context(), uuidToPGType(userUUID))
+			if err != nil {
+				log.Printf("User not found in database: %s", userIDStr)
+
+				// Clear invalid cookie
+				c.SetCookie(&http.Cookie{
+					Name:     "token",
+					Value:    "",
+					Expires:  time.Now().Add(-24 * time.Hour),
+					Path:     "/",
+					HttpOnly: true,
+					SameSite: http.SameSiteLaxMode,
+				})
+
+				// Redirect to login with session=invalid
+				return c.Redirect(http.StatusFound, "/login?session=invalid")
+			}
+
+			return next(c)
+		}
+	}
+}
+
 // wantsHTML determines if the request expects HTML response
 func wantsHTML(header http.Header) bool {
 	// Check Accept header
@@ -180,6 +221,16 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	registerDocumentationRoutes(cfg)
 	e.Static("/static", "web/static")
 
+	// Catch-all 404 handler - must be last
+	e.GET("/*", func(c echo.Context) error {
+		var buf bytes.Buffer
+		err := templates.ErrorPage("Page not found", "404").Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusNotFound, buf.String())
+	})
+
 	// Start background tasks (queue processor and connection cleanup)
 	scannerHandler.StartBackgroundTasks()