fix(frontend): disable New chat when the workspace has no providers

[willdady]

Summary

 <AppSidebar>
+  useHasNoProviders(scope)          // true only when the Providers read resolved empty
   <SidebarHeader>
-    <Button asChild><Link>New chat</Link></Button>
+    hasNoProviders
+      ? <Tooltip> <span tabIndex=0><Button disabled>New chat</Button></span>
+          "Add a provider to start a chat"              // canConfigureWorkspaceResource
+          "Ask an organization admin to add a provider" // otherwise
+      : <Button asChild><Link>New chat</Link></Button>

 <CommandMenu>
+  useHasNoProviders(scope)
-  <CommandItem>New chat</CommandItem>
+  <CommandItem disabled={hasNoProviders}>New chat</CommandItem>

Loading or errored Providers read → data undefined → New chat stays enabled. The Chat docs page now says New chat is unavailable until the Workspace has a Provider.

Closes #1428

Evidence

• Before: AppSidebar New chat > disables New chat once the workspace is known to have no providers fails (link to /chat rendered); CommandMenu > disables New chat… fails (item navigates).
  After: both pass, plus tooltip wording for org admin / delegated Workspace Owner / non-delegated Workspace Owner, and loading / errored / has-providers still navigating. pnpm lint, typecheck, format:check, test all green.

Merge Danger

Door: two-way

Blast Radius: small

One extra Providers read (SWR-deduped) in the sidebar and command menu. Worst case a stale empty cache briefly disables New chat until revalidation.

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