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/hooks/useDeviceContext.ts

@@ -0,0 +1,37 @@
+import { useState, useEffect } from 'react';
+
+export type DeviceType = 'desktop' | 'tablet' | 'phone' | 'iwb';
+
+function detectDeviceType(): DeviceType {
+  const width = window.innerWidth;
+  const touchPoints = navigator.maxTouchPoints ?? 0;
+  const hasTouch = touchPoints > 0 || 'ontouchstart' in window;
+
+  if (width >= 1280 && !hasTouch) return 'desktop';
+  if (width >= 768 && hasTouch) return 'tablet';
+  if (width < 768) return 'phone';
+  return 'desktop';
+}
+
+const STORAGE_KEY = 'cc_device_type';
+
+export function useDeviceContext() {
+  const [deviceType, setDeviceTypeState] = useState<DeviceType>(() => {
+    const stored = localStorage.getItem(STORAGE_KEY) as DeviceType | null;
+    if (stored && ['desktop', 'tablet', 'phone', 'iwb'].includes(stored)) return stored;
+    return detectDeviceType();
+  });
+
+  useEffect(() => {
+    localStorage.setItem(STORAGE_KEY, deviceType);
+  }, [deviceType]);
+
+  const setDeviceType = (type: DeviceType) => {
+    setDeviceTypeState(type);
+  };
+
+  const isTouch = deviceType === 'tablet' || deviceType === 'phone';
+  const isMobileLayout = deviceType === 'phone';
+
+  return { deviceType, setDeviceType, isTouch, isMobileLayout };
+}