v1.3 — Paper 26.1.x support + Kittle subfolder fix

Major
• Full Paper 26.1.x compatibility — new NMS module that targets the unified org.bukkit.craftbukkit.* package layout (no more v_X_RX suffix). Fixes the UnsupportedOperationException: Version v26_1 is not supported crash on the latest Paper builds. Also handles Mojang's split of ServerboundInteractPacket into separate interact / attack records.

Bug fixes
• Subfolders work again in TAB-complete and /images list — getImageFiles() now walks the images directory recursively, so users with hundreds of images organized in subfolders (pokemon/, signs/, etc.) get them listed and auto-completed properly. Big thanks to @Kittle for the report.
• Better "Invalid image name" feedback — three specific messages (cannot start with a dot / path traversal not allowed / illegal character) instead of the generic one, and forward-slashes in paths are normalized cross-platform. Thanks to @fett for raising it.
• Robust packet handler injection — the addBefore("packet_handler", ...) call is now wrapped with fallbacks (alternative pipeline names, then addLast as last resort), so newer Paper builds that rename the handler don't break image clicks.

Compatibility
• Adds 1.21.x → Paper 26.1.x. All previous versions (1.8 → 1.21.11) keep working.
• Java 25 only required for the Paper 26.1 module (the bundled JAR works on any Java the server supports).

Thanks
Special thanks to @OldComputerMan for prompting the 26.1.x update, @CloudeLecaw for confirming the crash trace, and everyone who tested intermediate builds.