feat(conv): choose which voice channel answers a chat widget's calls

[kmonsoe]

Why

When an organization has more than one voice channel, the chat widget's call button only works if the widget channel names one in voiceChannelId. Otherwise the server returns multiple_voice_channels_without_widget_routing instead of guessing. But nothing could set that field: no MCP tool, no /v1 route and no dashboard control. On top of that, conv_update_widget_channel rebuilt the stored config from a fixed list of fields, so any update, even one that only changed the origin allow-list, silently erased a link someone had set by hand.

What changes

Backend. Both MCP tools and the /v1 routes call WidgetChannelAdminService, so the change lands in one place.

• conv_create_widget_channel / POST /v1/conversations/channels/widget take an optional voiceChannelId.
• conv_update_widget_channel / PATCH /v1/conversations/channels/widget/:id take a nullable voiceChannelId: omit it to keep the link, pass null to unlink.
• The id is checked before anything is written. It must be a Vapi or Threll voice channel in the same organization that hasn't been deleted; a deactivated one is accepted because it can be reactivated. Anything else gets a 400 conv_widget_voice_channel_invalid with a code, so the dashboard shows it translated.
• Updates now start from the stored config, so fields the update doesn't mention are kept.
• The vendor list (vapi, threll) is one constant shared by the call-time lookup and the new check.

Dashboard (channels.tsx)

• Widget cards get an Edit dialog for the origin allow-list and the voice channel. The allow-list is only sent when it changed, so an older widget without one can still change its voice channel without tripping the allow-list requirement.
• The create dialog has the same voice channel picker. "Automatic" shows the channel's name when there is exactly one active voice channel. Deactivated channels are labelled, and a link to a deleted channel shows as "Removed channel".
• Each widget card says where its calls go, and gets the amber stripe when calls are off: several active voice channels with none picked, a deleted link, or a deactivated one. "Automatic" counts only active channels, matching the server.
• Edit takes the card's main button; the embed snippet sits beside it as a ghost button with no icon.
• New strings in en.json and nb.json.

Docs. setup-chat-widget.md gets a "Link a voice channel" section: how the server picks a channel, the reason codes, and the tool call. Fixtures and openapi.json are regenerated.

Tests

New cases in widget.integration.test.ts:

• linking, then checking the link survives unrelated updates (verified-identity toggle, allow-list edit), then unlinking with null;
• rejecting an SMS channel, a deleted voice channel, another org's voice channel, the widget itself and an unknown id, each time leaving the current link alone;
• the same link and rejection through PATCH /v1, which returns a 400 with the code;
• creating a widget already linked, and checking a rejected create leaves no channel behind.

Results:

• widget.integration.test.ts + widget-voice.integration.test.ts: 94 passed.
• dashboard-pages: 395 passed.
• Typecheck, eslint and the skill and PII checks are clean.
• I also ran the dashboard locally and checked the card footer layout in the browser.

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