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/router/reader.go

@@ -6,6 +6,7 @@ import (
 	"bookhoard/internal/services"
 	"bookhoard/templates"
 	"bytes"
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -73,7 +74,7 @@ func registerReaderRoutes(cfg *Config) {
 			MediaItemID: pgtype.UUID{Bytes: parsedUUID, Valid: true},
 			UserID:      uuidToPGType(userUUID),
 		})
-		if err != nil && err != pgx.ErrNoRows {
+		if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 			progress = database.ReadingProgress{}
 		}
 		// Get bookmarks