Let imports opt out of AI classification

Classification preferences gains "Classify newly imported transactions with
AI", stored as classify_on_import in config.toml and on by default, so existing
configurations keep their behaviour. It covers CSV imports and bank
synchronization alike.

With it off, no import path contacts the provider: classification falls to the
new provider-free rules path, where an opted-in merchant rule still applies its
category and tags, an alias match still attaches its merchant, and everything
else arrives on the editable fallback without a provenance error that would
suggest the provider had failed. Analyse remains available on demand.

web/src/api.ts

@@ -93,7 +93,11 @@ export interface State {
     banking_configured: boolean;
     ai_configured: boolean;
   };
-  settings: { model: string; include_amount: boolean };
+  settings: {
+    model: string;
+    include_amount: boolean;
+    classify_on_import: boolean;
+  };
   sessions: { session_id: string; valid_until: string; accounts: Account[] }[];
 }
 export interface Total {