Fix Flatpak build issues: EOL runtime and icon naming mismatch

- Bump the KDE runtime/SDK from 6.8 (now end-of-life per Flathub) to the
  current stable 6.10.
- The .desktop file's Icon=orbithub didn't match the app ID
  (io.orbithub.OrbitHub), so Flatpak's export step skipped it entirely
  ("Icon referenced in desktop file but not exported"). Use the app-ID-
  matching icon name everywhere instead, and drop the now-redundant
  renamed orbithub.svg install() rule that existed only for the old name.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/flatpak/io.orbithub.OrbitHub.yml

@@ -1,6 +1,6 @@
 app-id: io.orbithub.OrbitHub
 runtime: org.kde.Platform
-runtime-version: "6.8"
+runtime-version: "6.10"
 sdk: org.kde.Sdk
 command: orbithub
 finish-args: