docs: fix download links to 0.4.2 and optimize README screenshot

Point the GUI download buttons at the 0.4.2 assets (the 0.4.0 filenames
404'd) and shrink the README screenshot from 1.3 MB to ~170 KB
(1720px wide + pngquant) so the README loads fast.

Author: us

README.md

@@ -29,11 +29,11 @@
 
 | Platform | Download |
 |----------|----------|
-| **macOS (Apple Silicon)** | [Download .dmg](https://github.com/us/null-e/releases/latest/download/null-e_0.4.0_aarch64.dmg) — M1/M2/M3/M4 |
-| **macOS (Intel)** | [Download .dmg](https://github.com/us/null-e/releases/latest/download/null-e_0.4.0_x64.dmg) |
-| **Windows** | [Download .exe](https://github.com/us/null-e/releases/latest/download/null-e_0.4.0_x64-setup.exe) — 64-bit |
-| **Linux (Ubuntu/Debian)** | [Download .deb](https://github.com/us/null-e/releases/latest/download/null-e_0.4.0_amd64.deb) |
-| **Linux (Universal)** | [Download .AppImage](https://github.com/us/null-e/releases/latest/download/null-e_0.4.0_amd64.AppImage) |
+| **macOS (Apple Silicon)** | [Download .dmg](https://github.com/us/null-e/releases/latest/download/null-e_0.4.2_aarch64.dmg) — M1/M2/M3/M4 |
+| **macOS (Intel)** | [Download .dmg](https://github.com/us/null-e/releases/latest/download/null-e_0.4.2_x64.dmg) |
+| **Windows** | [Download .exe](https://github.com/us/null-e/releases/latest/download/null-e_0.4.2_x64-setup.exe) — 64-bit |
+| **Linux (Ubuntu/Debian)** | [Download .deb](https://github.com/us/null-e/releases/latest/download/null-e_0.4.2_amd64.deb) |
+| **Linux (Universal)** | [Download .AppImage](https://github.com/us/null-e/releases/latest/download/null-e_0.4.2_amd64.AppImage) |
 
 ### macOS Installation Note