Fix React startup crash by adding a resilient mock Supabase fallback when keys are missing

frontend/src/supabaseClient.js

@@ -3,4 +3,40 @@ import { createClient } from '@supabase/supabase-js'
 const supabaseUrl = import.meta.env.VITE_SUPABASE_URL
 const supabaseAnonKey = import.meta.env.VITE_SUPABASE_ANON_KEY
 
-export const supabase = createClient(supabaseUrl, supabaseAnonKey)
+const hasCredentials = supabaseUrl && supabaseAnonKey && 
+  supabaseUrl !== 'undefined' && supabaseAnonKey !== 'undefined';
+
+// Safe Chainable Thenable Promise to prevent React crashing on arbitrary queries
+const createChainablePromise = (resolvedValue) => {
+  const promise = Promise.resolve(resolvedValue);
+  const chain = {
+    select: () => chain,
+    order: () => chain,
+    eq: () => chain,
+    then: (onFulfilled, onRejected) => promise.then(onFulfilled, onRejected),
+    catch: (onRejected) => promise.catch(onRejected)
+  };
+  return chain;
+};
+
+// Safe Mock to allow sandbox-only local operations if DB keys aren't set
+const mockSupabase = {
+  from: () => ({
+    select: () => createChainablePromise({ data: [], error: null }),
+    insert: () => createChainablePromise({ data: null, error: new Error("Supabase credentials not configured in environment.") }),
+    update: () => ({ eq: () => createChainablePromise({ error: new Error("Supabase credentials not configured.") }) }),
+    delete: () => ({ eq: () => createChainablePromise({ error: new Error("Supabase credentials not configured.") }) })
+  }),
+  storage: {
+    from: () => ({
+      upload: () => createChainablePromise({ data: null, error: new Error("Supabase credentials not configured.") }),
+      remove: () => createChainablePromise({ error: new Error("Supabase credentials not configured.") }),
+      getPublicUrl: () => ({ data: { publicUrl: "" } })
+    })
+  }
+};
+
+export const supabase = hasCredentials 
+  ? createClient(supabaseUrl, supabaseAnonKey) 
+  : mockSupabase;
+