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/linux/io.orbithub.OrbitHub.desktop

@@ -5,7 +5,7 @@ Name=OrbitHub
 GenericName=Remote Session Manager
 Comment=Manage SSH and RDP sessions in one native app
 Exec=orbithub
-Icon=orbithub
+Icon=io.orbithub.OrbitHub
 Terminal=false
 Categories=Network;RemoteAccess;Utility;
 StartupNotify=true