fix(datagrid): keep the grid on screen while a table loads into a tab

[datlechin]

Why

Opening a table into the preview tab showed a blank pane until its rows arrived: no grid, no headers, only "Executing…" in the status bar. The retarget empties the tab's rows first, and since #2892 the pane drew "running, nothing loaded" as an empty Spacer, unmounting the grid.

Change

• A table keeps its grid while it loads. A pane with nothing to draw yet (a query's first run, JSON, Chart, Map) shows a delayed spinner. A dismissed load error no longer leaves a blank pane.
• A table opened into a tab no longer inherits the previous table's primary key, change marks or late metadata, so edits to a keyless table match the right rows.
• A SQL file's tab is not reused for a table, and a table that replaces one drops the file, so Save cannot write its query into it.
• Safe Mode (Full): Cancel no longer leaves Loading stuck, and an approval runs only on the tab it asked about. A tab restored behind another app now loads.

Verified

TableLoadKeepsGridUITests opens a slow view into the preview tab and checks the grid stays during the fetch. Unit tests cover the rest.

Screenshots

A slow view opened over customers, one second into the fetch.

	Light	Dark
Before
After

Not in this PR

• Redis database switching still replaces a tab holding typed SQL or edits.
• A table opened beside a kept tab skips its saved hidden columns and filters.
• LoadingReveal never reveals; fixing it adds a second status bar spinner.