feat(P4B.T3): useWhiteboardStrokes hook with INSERT + bulk DELETE realtime

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

apps/desktop/src/hooks/useWhiteboardStrokes.ts

@@ -0,0 +1,130 @@
+import { useCallback, useEffect, useState } from 'react';
+
+import {
+  clearWhiteboardStrokes,
+  insertWhiteboardStroke,
+  listWhiteboardStrokes,
+  type WhiteboardStroke,
+} from '@chat-app/shared/chat';
+
+import { supabase } from '../lib/supabase';
+
+interface State {
+  strokes: WhiteboardStroke[];
+  loading: boolean;
+  error: string | null;
+}
+
+export function useWhiteboardStrokes(whiteboardId: string | null): {
+  strokes: WhiteboardStroke[];
+  loading: boolean;
+  error: string | null;
+  insertStroke: (strokeJson: unknown) => Promise<void>;
+  clearAll: () => Promise<void>;
+} {
+  const [state, setState] = useState<State>({ strokes: [], loading: true, error: null });
+
+  useEffect(() => {
+    if (!whiteboardId) {
+      setState({ strokes: [], loading: false, error: null });
+      return;
+    }
+    let cancelled = false;
+    void (async () => {
+      try {
+        setState((s) => ({ ...s, loading: true, error: null }));
+        const list = await listWhiteboardStrokes(supabase, whiteboardId);
+        if (!cancelled) setState({ strokes: list, loading: false, error: null });
+      } catch (err) {
+        if (!cancelled) {
+          setState({
+            strokes: [],
+            loading: false,
+            error: err instanceof Error ? err.message : 'failed to load strokes',
+          });
+        }
+      }
+    })();
+    const channel = supabase
+      .channel('whiteboard:' + whiteboardId)
+      .on(
+        'postgres_changes',
+        {
+          event: 'INSERT',
+          schema: 'public',
+          table: 'whiteboard_strokes',
+          filter: 'whiteboard_id=eq.' + whiteboardId,
+        },
+        (payload) => {
+          const row = payload.new as {
+            id?: string;
+            whiteboard_id?: string;
+            author_user_id?: string;
+            stroke_json?: unknown;
+            created_at?: string;
+          } | null;
+          if (!row?.id || !row.whiteboard_id || !row.author_user_id || !row.created_at) return;
+          const next: WhiteboardStroke = {
+            id: row.id,
+            whiteboardId: row.whiteboard_id,
+            authorUserId: row.author_user_id,
+            strokeJson: row.stroke_json,
+            createdAt: row.created_at,
+          };
+          setState((s) => {
+            if (s.strokes.some((x) => x.id === next.id)) return s;
+            return { ...s, strokes: [...s.strokes, next] };
+          });
+        },
+      )
+      .on(
+        'postgres_changes',
+        {
+          event: 'DELETE',
+          schema: 'public',
+          table: 'whiteboard_strokes',
+          filter: 'whiteboard_id=eq.' + whiteboardId,
+        },
+        () => {
+          // Bulk delete via "Clear all" — drop everything; future inserts
+          // come back via the INSERT branch above.
+          setState((s) => ({ ...s, strokes: [] }));
+        },
+      )
+      .subscribe();
+    return () => {
+      cancelled = true;
+      void supabase.removeChannel(channel);
+    };
+  }, [whiteboardId]);
+
+  const insertStroke = useCallback(
+    async (strokeJson: unknown) => {
+      if (!whiteboardId) return;
+      try {
+        await insertWhiteboardStroke(supabase, { whiteboardId, strokeJson });
+        // No optimistic append — realtime echoes the row back in <150ms.
+      } catch (err) {
+        console.error('insertWhiteboardStroke failed', err);
+        setState((s) => ({
+          ...s,
+          error: err instanceof Error ? err.message : 'stroke insert failed',
+        }));
+      }
+    },
+    [whiteboardId],
+  );
+
+  const clearAll = useCallback(async () => {
+    if (!whiteboardId) return;
+    await clearWhiteboardStrokes(supabase, whiteboardId);
+  }, [whiteboardId]);
+
+  return {
+    strokes: state.strokes,
+    loading: state.loading,
+    error: state.error,
+    insertStroke,
+    clearAll,
+  };
+}