feat(query): sql workspace conveniences per opsxdb patterns

- generic ContextMenu component (viewport flip, esc/outside close)
- SchemaSidebar table right-click menu: preview data, generate
  SELECT/INSERT/UPDATE/DELETE templates with pk-aware WHERE, copy table
  name; double-click table runs preview SELECT; column right-click
  copy/insert; dialect-aware quoting; preview SELECT carries no LIMIT
- SqlEditor selection-first execution (Mod-Enter runs selection else
  statement at cursor via quote/comment-aware splitter); pagination
  replays last executed statement
- query history persisted server-side: drawer fetches query-histories
  with keyword/mine/favorites filters, star toggle (new PUT favorite +
  mine filter endpoints), insert and insert-and-run
- custom templates in localStorage alongside built-ins
- unit tests: genSql + statement splitter (28 tests green)

frontend/src/lib/storage.ts

@@ -36,6 +36,7 @@ export const STORAGE_KEYS = {
   theme: 'opsxsql.theme.mode',
   loginStyle: 'opsxsql.ui.loginStyle',
   queryFavorites: 'opsxsql.query.favorites',
+  queryCustomTemplates: 'opsxsql.query.customTemplates',
   querySideWidth: 'opsxsql.query.sideWidth',
   chunkReloaded: 'opsxsql.app.chunkReloaded',
 } as const;