refactor: use errors.Is()/errors.AsType() for error comparison and rename shadowed variables

Replace direct error equality checks (err == pgx.ErrNoRows, err != http.ErrServerClosed)
with the idiomatic errors.Is() function throughout handlers, services, middleware, and app
startup. This correctly handles wrapped error chains.

Also replace a raw type assertion (*HTTPError) with errors.AsType[*HTTPError]() in the
error handler middleware for consistency.

Additionally, rename shadowed variables for clarity:
- sidecar.go: config -> sidecarConfig, systemConfig (shadowed package-level vars)
- media_scanner.go: uuid -> uuidString (shadowed the uuid package import)

internal/handlers/collections.go

@@ -6,6 +6,7 @@ import (
 	wsync "bookhoard/internal/sync"
 	"bookhoard/internal/utils"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -891,7 +892,7 @@ func (h *CollectionHandler) PreviewCollection(c *echo.Context) error {
 
 	_, err = h.db.GetLibrary(c.Request().Context(), pgtype.UUID{Bytes: libUUID, Valid: true})
 	if err != nil {
-		if err == pgx.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return c.JSON(http.StatusBadRequest, map[string]string{"error": "library not found"})
 		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})