VoiceOS: background tools skip preToolUse hooks; confirm widgets see only args

In VoiceOS 0.1.40 (Electron), integration tools declared execution.mode background go straight to the MCP execute call, and the preToolUse hook (voiceos_hook_pre_tool_use, manifest hooks.preToolUse scope own) only runs on the sync path, so a hook can't rewrite or enrich a background write's args. A manifest confirmation widget gets voiceos:init with args (the pending call) plus data, but data is the static value from the manifest, not per call. Any per-call image (for example a video preview) has to travel as a tool argument the model copies, so keep it a compact string (a DCT/ThumbHash-style encoding of about 100 chars) and draw it on a canvas, since the widget has no network. Result widgets can embed data: images up to about 300K chars of HTML.