Allow users to resize the custom panel by dragging a handle on its left
edge, matching the chat panel's resize UX. The panel width is persisted
via PersistenceRegistry so it survives page reloads.
Architectural changes:
- Split CustomPanel into container (resize/close) + CustomPanelContent (branding override)
- Split functions.ts into core functions + functions.custom.ts (branding override)
- Split middleware.web.ts into orchestrator + middleware.custom.web.ts (branding override)
- Chat max size now accounts for custom panel width for mutual awareness
test-localStorage1.html
