fix(datagrid): redraw cell when editability changes (chevron flicker on save/refresh)

[datlechin]

Summary

DataGridCellView.computeAccessoryRect gates the chevron accessory on isEditableCell for dropdown / boolean / date / JSON / blob cells:

```swift
case .dropdown, .boolean, .date, .json, .blob:
guard isEditableCell else { return .zero }
```

After #1212's performance pass, `configure` only marks the cell `needsDisplay = true` when a tracked property changed. `isEditableCell` was updated unconditionally:

```swift
isEditableCell = state.isEditable
```

…so when only the editability flipped (refresh, safe-mode toggle, view-to-table switch) with otherwise identical cell content, no redraw was scheduled and the chevron stuck in its stale state. User saw `audit_log.payload` (LONGTEXT with `json_valid` check, detected as JSON) chevron disappear after save / refresh / reopen.

Fix

Compare before assign and flag `needsRedraw` on change:

```swift
if isEditableCell != state.isEditable {
isEditableCell = state.isEditable
needsRedraw = true
}
```

Audited the rest of `configure`. Other unconditional assignments (`placeholder`, `isLargeDataset`) are write-only fields that aren't read elsewhere, so they can't affect drawing. `cellRow` / `cellColumnIndex` only feed accessibility. The two cells that do affect drawing (text/tint/visualState/kind/rawValue/focus) were already tracked.

Test plan

• Open MySQL `tablepro_demo` → `audit_log`. `payload` column shows chevron on each cell.
• Hit Refresh. Chevron stays visible.
• Toggle Safe Mode (read-only). Chevrons disappear immediately.
• Toggle Safe Mode back off. Chevrons reappear.
• Open a view (e.g. `active_users`) then back to `audit_log`. Chevron state correct.
• Same check on dropdown columns (`status` ENUM in `wide_events`) and date columns (`event_date`, `occurred_at`).