feat(grid): in-place editing in Table Browser (Postgres/MySQL/SQLite) #759

Summary

Table Browser (tree → table) is still a read-only grid. SQL Workspace already has DML staging (DataGridStagingBuffer + TableMutationEngine). Same grid, different behavior — 0.4.16 P0.

Scope

• Wire DataGridStagingBuffer into Postgres / MySQL / SQLite table views via ResultsTab.
• Load PRIMARY KEY metadata (getTableSchema) on browse; disable editing for views, materialized views, custom SQL, and tables with no PK (with a clear status message).
• Apply staged edits transactionally through TableMutationEngine + DML preview.
• Toolbar: pending-change badge, Save / Revert next to pagination; block Prev/Next while dirty.
• Confirm before Refresh / leaving the browse page with unsaved cell edits.

Out of scope

• MongoDB document editor (0.4.16 P2).
• README Feature Matrix copy (P2).

Acceptance

• Double-click a cell in Table Browser opens the inline editor when a PK exists.
• Save generates correct DML and commits in a transaction.
• Views / no-PK tables stay read-only with an explanation.
• Unsaved edits warn on refresh / home navigation; pagination is blocked while dirty.
• flutter test / flutter analyze clean.

Refs: 0.4.16 planning, SQL Workspace staging (0.4.13).