port to Minecraft 26.3

No source changes were needed — nothing JustBackup touches moved in 26.3.
Version bumps only:

- minecraft 26.2 -> 26.3, loader 0.19.3 -> 0.19.5
- fabric-api 0.154.2+26.2 -> 0.160.5+26.3
- fabric.mod.json depends: minecraft ~26.2 -> ~26.3,
  fabricloader >=0.19.3 -> >=0.19.5
- README version line

Verified with `./gradlew build` against 26.3 (clean).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gradle.properties

@@ -7,8 +7,8 @@ org.gradle.configuration-cache=false
 
 # Fabric Properties
 # check these on https://fabricmc.net/develop
-minecraft_version=26.2
-loader_version=0.19.3
+minecraft_version=26.3
+loader_version=0.19.5
 loom_version=1.17-SNAPSHOT
 
 # Mod Properties
@@ -17,4 +17,4 @@ maven_group=io.ib67.sfcraft
 archives_base_name=justbackup
 
 # Dependencies
-fabric_version=0.154.2+26.2
+fabric_version=0.160.5+26.3