feat(web): add posthog event for chat preferences saved

Registers `wa_chat_preferences_saved` in `PosthogEventMap` and fires it
on a successful save from the Chat Preferences page. The `wa_` prefix
is correct here: the event can only originate from the web app.

Properties capture adoption signals without leaking the actual
preference content:
- `dimensionsSet`: count of dimensions the user has chosen (0-6)
- `hasCustomInstructions`: boolean flag for free-text usage
- `customInstructionsLength`: length of the trimmed custom instructions
  so we can later distinguish drive-by notes from heavy use

No preference values, level names, or instruction text are sent.

Refs #1242

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: adam.wilson

packages/web/src/app/(app)/settings/chatPreferences/chatPreferencesPage.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { useToast } from "@/components/hooks/use-toast";
+import useCaptureEvent from "@/hooks/useCaptureEvent";
 import { LoadingButton } from "@/components/ui/loading-button";
 import { Textarea } from "@/components/ui/textarea";
 import { ToggleGroup, ToggleGroupItem } from "@/components/ui/toggle-group";
@@ -25,6 +26,7 @@ export function ChatPreferencesPage({
     initialCustomInstructions,
 }: ChatPreferencesPageProps) {
     const { toast } = useToast();
+    const captureEvent = useCaptureEvent();
 
     const [preferences, setPreferences] = useState<ChatPreferences>(initialPreferences);
     const [customInstructions, setCustomInstructions] = useState<string>(initialCustomInstructions ?? "");
@@ -102,6 +104,11 @@ export function ChatPreferencesPage({
                 preferences,
                 customInstructions,
             });
+            captureEvent("wa_chat_preferences_saved", {
+                dimensionsSet: Object.keys(preferences).length,
+                hasCustomInstructions: trimmedCustom.length > 0,
+                customInstructionsLength: trimmedCustom.length,
+            });
             toast({
                 title: "Chat preferences saved",
                 description: "Sourcebot will apply these to future chats.",
@@ -115,7 +122,7 @@ export function ChatPreferencesPage({
         } finally {
             setIsSaving(false);
         }
-    }, [preferences, customInstructions, isOverLimit, toast]);
+    }, [preferences, customInstructions, isOverLimit, toast, captureEvent]);
 
     return (
         <div className="flex flex-col gap-8">

packages/web/src/lib/posthogEvents.ts

@@ -185,6 +185,14 @@ export type PosthogEventMap = {
         chatId: string,
         currentVisibility: 'PUBLIC' | 'PRIVATE',
     },
+    wa_chat_preferences_saved: {
+        /** Number of preference dimensions the user has set (0-6). */
+        dimensionsSet: number,
+        /** True if the user supplied any non-empty custom instructions. */
+        hasCustomInstructions: boolean,
+        /** Length of the custom instructions string, for adoption sizing. */
+        customInstructionsLength: number,
+    },
     wa_chat_visibility_changed: {
         chatId: string,
         fromVisibility: 'PUBLIC' | 'PRIVATE',