feat: add OPDS server reordering with sortOrder field and numeric stepper

Servers are sorted by a persistent sortOrder field (ties broken
alphabetically). On-device editing uses a new NumericStepperActivity
with side buttons for ±1 and face buttons for ±10. The web UI gets
up/down arrow buttons and a POST /api/opds/reorder endpoint.

Made-with: Cursor

lib/I18n/I18nKeys.h

@@ -426,6 +426,7 @@ enum class StrId : uint16_t {
   STR_SAVE_HERE,
   STR_SELECT_FOLDER,
   STR_DOWNLOAD_PATH,
+  STR_POSITION,
   // Sentinel - must be last
   _COUNT
 };