feat(settings): protect model provider deletion and cascade credential updates

- Block deletion of a model provider when it is referenced by any agent,
  returning an error that lists the agent names so the user knows what to unlink first
- When a provider's api_url or api_key is updated, automatically propagate
  the new credentials to all dependent agents: updates env_json in the database
  and patches on-disk config files (Claude Code settings.json, Gemini settings.json,
  Codex auth.json + config.toml, OpenCode auth.json) using the same field names
  and structure as the agent settings UI
- Fix error message display in provider dialogs for both Tauri and web transports,
  which throw plain objects rather than Error instances

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/settings/edit-model-provider-dialog.tsx

@@ -97,8 +97,14 @@ export function EditModelProviderDialog({
       toast.success(t("editSuccess"))
       handleOpenChange(false)
       onProviderUpdated()
-    } catch (err) {
-      const msg = err instanceof Error ? err.message : String(err)
+    } catch (err: unknown) {
+      const raw = err as Record<string, unknown>
+      const msg =
+        typeof raw?.message === "string"
+          ? raw.message
+          : err instanceof Error
+            ? err.message
+            : String(err)
       setError(msg)
     } finally {
       setLoading(false)