feat: user preferences sync — server-side storage, whiteboard defaults, dashboard widget visibility

- New table `user_preferences` (user_id PK, JSON blob, updated_at)
- GET/PATCH/DELETE /api/preferences with deep-merge UPSERT
- LS.prefs singleton in api.js: dot-notation get/set, debounced flush (1.5s), server sync
- classroom.html: load wb.color/width/lineStyle/theme from prefs on init; save on change
- dashboard.html: widget configurator panel (gear button) — toggle visibility per-user, persisted server-side

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

backend/src/db/migrate.js

@@ -2898,4 +2898,13 @@ db.exec(`
   )
 `);
 db.exec('CREATE INDEX IF NOT EXISTS idx_geo_subs_task ON geometry_submissions(task_id)');
+
+// User preferences (server-synced: whiteboard defaults, dashboard visibility, etc.)
+db.exec(`
+  CREATE TABLE IF NOT EXISTS user_preferences (
+    user_id    INTEGER PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    data       TEXT    NOT NULL DEFAULT '{}',
+    updated_at TEXT    NOT NULL DEFAULT (datetime('now'))
+  )
+`);
 db.exec('CREATE INDEX IF NOT EXISTS idx_geo_subs_student ON geometry_submissions(student_id)');