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.

migrations/1787515927.sh

@@ -0,0 +1,27 @@
+echo "Stop world-writable Chromium and Firefox policy directories"
+
+source "$OMARCHY_PATH/install/helpers/browser-policy.sh"
+
+repaired=0
+for dir in "${BROWSER_POLICY_MANAGED_DIRS[@]}"; do
+  [[ -d $dir || -L $dir ]] || continue
+  browser_policy_setup_dir "$dir"
+  repaired=1
+done
+
+if (( repaired )); then
+  omarchy-theme-set-browser
+fi
+
+for dir in "${BROWSER_POLICY_FIREFOX_DIRS[@]}"; do
+  [[ -d $dir || -L $dir ]] || continue
+  if browser_policy_firefox_hardened "$dir"; then
+    browser_policy_purge_dir "$dir"
+    continue
+  fi
+  browser_policy_setup_parent "$dir"
+  browser_policy_purge_dir "$dir"
+  if ! browser_policy_firefox_policy_file_ok "$dir/policies.json"; then
+    browser_policy_install_firefox_policies "$dir"
+  fi
+done