feat: standalone LvScreenshot library v1.0.0

Promoted from DIYStickyNotes/lib/LvScreenshot (byte-identical src) into a
reusable Gitea library. Adds LICENSE, .gitignore, library.properties,
keywords.txt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

library.json

@@ -0,0 +1,11 @@
+{
+  "name": "LvScreenshot",
+  "version": "1.0.0",
+  "description": "Board-independent LVGL 9 screen capture (lv_snapshot_take -> tight RGB565) plus an optional AsyncWebServer /screenshot endpoint and a PC tool that saves a PNG.",
+  "keywords": "lvgl, screenshot, snapshot, capture, png, esp32",
+  "frameworks": "arduino",
+  "platforms": "espressif32",
+  "headers": "LvScreenshot.hpp",
+  "license": "MIT",
+  "build": { "srcDir": "src" }
+}