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>

test/shell.d/browser-env-test.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(dirname "$0")/base-test.sh"
+
+envs="$ROOT/default/bash/envs"
+uwsm_default="$ROOT/default/uwsm/default"
+uwsm_env="$ROOT/default/uwsm/env.d/10-omarchy"
+
+browser=$(env -u BROWSER bash -c 'source "$1"; printf "%s" "$BROWSER"' bash "$envs")
+[[ $browser == "omarchy-launch-browser" ]] || fail "bash env provides a default browser" "actual: $browser"
+pass "bash env provides a default browser"
+
+browser=$(BROWSER=firefox bash -c 'source "$1"; printf "%s" "$BROWSER"' bash "$envs")
+[[ $browser == "firefox" ]] || fail "bash env preserves the inherited browser" "actual: $browser"
+pass "bash env preserves the inherited browser"
+
+# A session-wide BROWSER makes xdg-settings refuse "set default-web-browser",
+# breaking the browsers' own "Set as default" buttons.
+browser=$(env -u BROWSER bash -c 'source "$1"; printf "%s" "${BROWSER:-}"' bash "$uwsm_default")
+[[ -z $browser ]] || fail "uwsm session env leaves BROWSER unset" "actual: $browser"
+pass "uwsm session env leaves BROWSER unset"
+
+! grep -q "export BROWSER" "$uwsm_env" || fail "uwsm env.d fallback leaves BROWSER unset"
+pass "uwsm env.d fallback leaves BROWSER unset"