[Bug]: Cannot select or copy rows from a read-only result grid

| | Do | Expected | Actual |
|---|---|---|---|
| 1 | Left-click a cell | the row is selected | a text editor is placed over the cell, covering the value |
| 2 | Right-click in that cell | the grid's context menu | the platform's text menu (Undo / Cut / Paste). The grid's own menu cannot be reached |
| 3 | Right-click a row, outside the editor | usable actions | *Select all rows*, *Clear selection*, *Filtered selection* and *Copy selected lines to clipboard* are all disabled |
| 4 | Copy a single cell value | some way to do it | there is none. The nearest workaround is selecting text inside the editor from step 1 |

The same happens in every read-only grid, including the SQL results and the execution views.

## Expected behavior and proposed change

A read-only result grid should prevent changes to the data while still allowing users to inspect,
select and copy it.

- Clicking a cell should select its row without opening an inline editor.
- Right-clicking should open the grid's context menu.
- Selection actions and copying selected rows should remain available.
- A **Copy cell value** action should copy the complete underlying value, even when the displayed
text is shortened.

Long values must remain accessible for viewing and copying after the inline editor is removed — the
pop-out value viewer stays reachable from the keyboard, and *Copy cell value* takes the stored value,
not the shortened display string.

## Cause

All of it is in `TableView`:

- `editSelected` calls `edit(...)` on a left click and only checks `table.isEnabled()`; nothing
consults `readonly`. On a read-only grid the editor has nothing to edit.
- Four menu items are switched off where they are created, with `setEnabled(!readonly)`:
*Select all rows*, *Clear selection*, *Filtered selection* and *Copy selected lines to clipboard*.
None of them changes the table — `setFilter()` ends in `table.setSelection(...)`, `clipSelected()`
reads.
- `clipSelected()` copies whole rows with a header line; there is no single-cell copy.

The result grid built by `DatabaseResultsPanel` shows all three at once: it is read-only, and it
uses `setShortenDisplayedValues(true)`, so long values are displayed shortened. That is why a
single-cell copy has to take the stored value rather than the display string.

## Scope of the fix

One class — `TableView.java` — plus one message key:

1. `editSelected`: when `readonly`, select the row and return.
2. Drop the `!readonly` gate on the four menu items.
3. Add *Copy cell value* beside *Copy selected lines to clipboard*, using `getCellValue` for the
stored value.

Happy to open the PR if the approach looks right. The one point worth an opinion is whether to
suppress the inline editor on read-only grids, or to keep it and give the grid's menu precedence over
it.

### Issue Priority

Priority: 3

### Issue Component