feat(phase-a): remove Neo4j from startup chain, clean auth flow

- Remove NeoUserProvider + NeoInstituteProvider from App.tsx startup chain
- Strip user_db_name/school_db_name from CCUser; add school_id (Phase B wires it to Supabase)
- Remove DatabaseNameService from AuthContext and UserContext
- Remove provisionUser() call from login path; API endpoint preserved for Phase B decision
- Simplify UserContext.resolveProfile: fast-path JWT metadata then background Supabase fetch
- Replace user.user_db_name reads in singlePlayerPage + snapshotService with null-safe guards
- Add useDeviceContext hook (desktop/tablet/phone/iwb, persists to localStorage)

App now loads to dashboard without any Neo4j dependency at startup.
Canvas opens to blank TLDraw state; Phase B rebuilds navigation on Supabase.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/contexts/AuthContext.tsx

@@ -4,7 +4,6 @@ import { Session, User } from '@supabase/supabase-js';
 import { CCUser, CCUserMetadata, authService } from '../services/auth/authService';
 import { logger } from '../debugConfig';
 import { supabase } from '../supabaseClient';
-import { DatabaseNameService } from '../services/graph/databaseNameService';
 import { storageService, StorageKeys } from '../services/auth/localStorageService';
 
 export interface AuthContextType {
@@ -52,20 +51,13 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
     const baseDisplayName = metadata.display_name || metadata.name || metadata.preferred_username || baseUsername;
     const userType = (metadata.user_type || 'email_teacher').trim();
 
-    const storedUserDb = DatabaseNameService.getStoredUserDatabase();
-    const storedSchoolDb = DatabaseNameService.getStoredSchoolDatabase();
-
-    const userDbName = storedUserDb || DatabaseNameService.getUserPrivateDB(userType || 'standard', supabaseUser.id);
-    const schoolDbName = storedSchoolDb || '';
-
     const resolvedUser: CCUser = {
       id: supabaseUser.id,
       email: supabaseUser.email,
       user_type: userType,
       username: baseUsername,
       display_name: baseDisplayName,
-      user_db_name: userDbName,
-      school_db_name: schoolDbName,
+      school_id: null,
       created_at: supabaseUser.created_at,
       updated_at: supabaseUser.updated_at
     };