Scope BROWSER to interactive shells so xdg-settings can change the default browser (#6616)

Exporting BROWSER=omarchy-launch-browser into the whole uwsm session made
xdg-settings refuse "set default-web-browser", which broke every browser's
own "Set as default" button. The export only exists for terminal programs
(like gh) to open URLs detached from the terminal process tree, so move it
to default/bash/envs where interactive shells still pick it up.

Fixes #6590

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

default/uwsm/env.d/10-omarchy

@@ -9,7 +9,6 @@ if [ -f "${OMARCHY_PATH%/}/default/uwsm/default" ]; then
   . "${OMARCHY_PATH%/}/default/uwsm/default"
 else
   export TERMINAL=xdg-terminal-exec
-  export BROWSER=omarchy-launch-browser
   export EDITOR="omarchy-launch-editor --inline"
 fi