Summary

• Table Browser (Postgres, MySQL, SQLite) now uses the same DataGridStagingBuffer + ResultsTab path as SQL Workspace, so double-click edits cells when a PRIMARY KEY exists.
• Views / materialized views / custom SQL / tables without a PK stay read-only with a status-line explanation. Save runs a DML preview then a transaction.
• Toolbar shows a pending-change badge with Save / Revert; Prev/Next are blocked while dirty; Refresh and Server/Overview confirm before discarding edits.

Closes #759

Test plan

• Open a Postgres table with a PK from the tree — double-click a cell, stage an edit, Save (preview SQL, commit, toast).
• Confirm views and a table without a PK show the read-only reason and do not open the editor.
• Stage edits, then Prev/Next stay disabled; Refresh and Server prompt to discard.
• Repeat a save on MySQL and SQLite tables with a PK.
• flutter test test/features/workspace/table_view_staging_test.dart test/features/postgresql/postgres_table_toolbar_layout_test.dart