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/add-model-provider-dialog.tsx

@@ -89,8 +89,14 @@ export function AddModelProviderDialog({
       toast.success(t("createSuccess"))
       handleOpenChange(false)
       onProviderAdded()
-    } 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)

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)

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

@@ -72,9 +72,21 @@ export function ModelProviderSettings() {
       toast.success(t("deleteSuccess"))
       setDeleteTarget(null)
       await loadProviders()
-    } catch (err) {
-      const msg = err instanceof Error ? err.message : String(err)
-      toast.error(msg)
+    } 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)
+      const prefix = "PROVIDER_IN_USE:"
+      if (msg.includes(prefix)) {
+        const agentNames = msg.substring(msg.indexOf(prefix) + prefix.length)
+        toast.error(t("deleteBlockedByAgent", { agents: agentNames }))
+      } else {
+        toast.error(msg)
+      }
     }
   }, [deleteTarget, loadProviders, t])