submodule(hbb_common): rebase pro-features onto upstream/main

Rebases the fork's branch onto current upstream rustdesk/hbb_common main
(6490a86), pulling in 267 upstream commits (security hardening, IPC perm
fixes, file-transfer path-traversal fixes, privacy-mode option, etc).
The local HttpProxyRequest/Response backport is dropped because upstream
now has the same proto messages with the same tag numbers.

Cherry-picks the local "implement app_icon" commit on top, adding the
OPTION_APP_ICON buildin key needed by the customization page.

Sets branch=pro-features in .gitmodules so future
'git submodule update --remote' tracks the right branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "libs/hbb_common"]
 	path = libs/hbb_common
 	url = https://gitea.cstudio.ch/mike/hbb_common.git
+	branch = pro-features