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/services/tldraw/snapshotService.ts

@@ -281,8 +281,12 @@ export class NavigationSnapshotService {
                 throw new Error('No user found');
             }
 
-            const dbName = user.user_db_name;
-            
+            const dbName = (user as (typeof user & { user_db_name?: string })).user_db_name ?? '';
+            if (!dbName) {
+                logger.debug('snapshot-service', '⚠️ No db name - snapshot save skipped (Phase B will migrate to Supabase Storage)');
+                return;
+            }
+
             logger.debug('snapshot-service', '💾 Saving snapshot', {
                 nodePath,
                 dbName,
@@ -315,8 +319,12 @@ export class NavigationSnapshotService {
                 throw new Error('No user found');
             }
 
-            const dbName = user.user_db_name;
-            
+            const dbName = (user as (typeof user & { user_db_name?: string })).user_db_name ?? '';
+            if (!dbName) {
+                logger.debug('snapshot-service', '⚠️ No db name - snapshot load skipped (Phase B will migrate to Supabase Storage)');
+                return;
+            }
+
             logger.debug('snapshot-service', '📥 Loading snapshot', {
                 nodePath: node.node_storage_path,
                 dbName,