fix(linux): use %U in Dolphin service menu

[simonbcn]

What do you want to change?

• File: dist/linux/ghostty_dolphin.desktop
• Change: Exec=ghostty +new-window --working-directory=%F → %U
• One character, no code involved

Why do you want to make this change?

• Dolphin shows inode/directory service menus in two places: on a selected folder, and on the empty view background
• Selected folder: %F expands fine, ghostty opens in that folder
• Empty background: %F expands to nothing → --working-directory= empty → ghostty falls back to $HOME
• %U works in both: KIO passes the plain local path, already percent-decoded, no file:// prefix (measured: /home/juan/Documentos)
• Tested: ghostty 1.3.1, Dolphin 26.04.3, KIO 6.28.0, Plasma 6.7.3, Wayland

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[jcollie]

!vouch