Sync TableView SelectedItems with the always-selected row 0 (#13230)

[niksedk]

Fixes #13230.

The bug

After opening a subtitle, the first row looks selected and its text shows - then a few seconds later the text box, Show and Duration go blank while row 1 stays highlighted. A shift-selection that visually starts at row 1 also leaves row 1 out, so italic/copy/delete skip the first line.

Root cause

MakeTableView creates every grid with SelectionMode.AlwaysSelected. In Avalonia 12.1.1 that mode selects row 0 as soon as ItemsSource is assigned to a populated collection, but the pick only reaches the internal selection model - the row is drawn highlighted and SelectedItem/SelectedIndex point at it, while the SelectedItems collection stays empty and no SelectionChanged is raised. Verified headlessly:

after ItemsSource=items:  SelectedItem=line 1  SelectedIndex=0  Selection.Count=1
                          SelectedItems.Count=0        <-- desync
events: <none>

It does not heal on a layout pass, and neither Selection.Clear()+Select(0) nor an ItemsSource/SelectedIndex bounce repairs it - only moving the selection to a different row does, which is why clicking a row makes everything work again. Without AlwaysSelected the same sequence stays in sync, so the mode is the trigger.

Subtitle Edit reads every selection through SubtitleGridSelectedItems -> SubtitleGrid.SelectedItems, so the whole application saw nothing selected:

• Blank edit box a few seconds in - the two-way SelectedItem binding does push row 0 into SelectedSubtitle, so the text shows at first. On the open-last-file-on-start path the restore chain ends in SetRecentFileProperties -> UpdateVideoOffsetStatus -> SubtitleGridSelectionChanged, which reads the empty SelectedItems, takes the "nothing selected" branch and sets SelectedSubtitle = null. That call runs after the awaited video open, which is the several-second delay - and it is why the repro needs a close-and-reopen rather than a plain File > Open.
• Row 1 omitted from a selection - reproduced exactly. Applying the grid's own shift-select range over rows 1-5 yields Selection.Count=5 but SelectedItems=[line 2, line 3, line 4, line 5].

The #13190 fix does not cover this: its guard is !ReferenceEquals(SelectedSubtitle, itemToScroll), and the two-way binding has already set SelectedSubtitle, so it never fires - and if it did it would read the same empty SelectedItems.

The fix

Repair SelectedItems from the selection model on every ItemsSource change, in MakeTableView so every grid in the application is covered rather than the main subtitle grid alone. The repair is a no-op unless SelectedItems is empty while the selection model is not.

Tests

Four regression tests in tests/UI/Logic/TableViewSelectionSyncTests.cs, built on a grid configured like the main subtitle grid (multi-select, always-selected, two-way SelectedItem/SelectedIndex bindings) and filled the way SetSubtitles fills it. Three of them fail without the fix; the fourth is the empty-grid guard case. Full UI suite: 1417 passing.

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