Harden browser policy directories (#7972)

Cherry-picked from quattro (7d58bb9a).

Stop world-writable Chromium and Firefox policy directories: create them
root-owned at 0755, purge non-root entries, refuse planted symlinks, and
write the browser theme colour through a passwordless helper instead of
a world-writable policy file.

Conflict resolution for v4-0-2:
- bin/omarchy-install-browser: dropped the `chromium)` case, which does
  not exist on this branch.
- test/shell.d/default-apps-test.sh: dropped; the file does not exist on
  this branch.

install/config/theme-system.sh

@@ -6,10 +6,6 @@ ln -snf /usr/share/icons/Adwaita/symbolic/actions/go-next-symbolic.svg \
           /usr/share/icons/Yaru/scalable/actions/go-next-symbolic.svg
 gtk-update-icon-cache /usr/share/icons/Yaru &>/dev/null || true
 
-# Chromium policy directory for theme
-mkdir -p /etc/chromium/policies/managed
-chmod a+rw /etc/chromium/policies/managed
-
 # Default Chromium to follow system appearance ("device") instead of dark
 mkdir -p /usr/lib/chromium
 echo '{"browser":{"theme":{"color_scheme":0,"color_scheme2":0}}}' > \