feat: enable/disable providers from settings

Adds a per-provider toggle in Settings → Providers, persisted via daemon
config, so users can turn off providers they don't have installed (or
don't want to use) without removing them from the registry.

Disabled providers stay visible everywhere — settings, `paseo provider
ls`, MCP `list_providers`, model/mode lists — marked as disabled rather
than silently filtered. Availability probes are skipped for them, and
attempts to launch an agent or fetch models/modes from a disabled
provider return a clear "Provider '<id>' is disabled" error.

Schema additions (`enabled` on `ProviderSnapshotEntry`, MCP provider
summary) are backward-compatible: optional with a default of true so
older clients continue to parse new daemon messages.

CLI: `paseo provider ls` gains an ENABLED column; JSON output includes
the new field.

packages/app/src/hooks/use-agent-form-state.ts

@@ -396,7 +396,7 @@ function buildProviderDefinitionMapForStatuses(args: {
 
   const matchingProviders = new Set(
     args.snapshotEntries
-      .filter((entry) => args.statuses.has(entry.status))
+      .filter((entry) => args.statuses.has(entry.status) && entry.enabled !== false)
       .map((entry) => entry.provider),
   );