fix: focus the tile pane when clicking its empty canvas (0.8.3-beta1)

[piranout]

What

Usability bug: after clicking a folder in the tree, a click on the empty canvas of the tile/contents pane did not move keyboard focus. Arrows, PageUp/PageDown, and scroll keys stayed on the tree. Clicking a tile already transferred focus (the ListBoxItem focuses itself). Canvas should do the same.

Cause (verified): the ListBox is Focusable, but WPF only focuses a tile because ListBoxItem calls Focus() on mouse-down. An empty-canvas click hits chrome that never claims keyboard focus, so the tree keeps it. Not a missing Focusable flag on the ListBox itself.

Fix (pane-level): BrowserHost is hit-testable (Background="Transparent") and handles PreviewMouseLeftButtonDown. Canvas clicks call ItemsList.Focus(). One decision path (LibraryPaneFocus) skips:

• tiles (they already focus the item)
• section headers (ToggleButton already focuses)
• text boxes (search / rename / notes keep the caret)
• dialogs

Tree-click behavior is unchanged (handler is only on the contents pane). Tile-click behavior is unchanged.

Version is 0.8.3-beta1 in Aperture.App.csproj so merge re-exercises the auto-tag/release path. CHANGELOG has ## [0.8.3-beta1].

How to test

1. dotnet test tests/Aperture.Core.Tests/Aperture.Core.Tests.csproj — includes LibraryPaneFocusTests (the claim/skip contract). Live WPF focus is not unit-tested (no dispatcher circus).
2. Manual click path (this is the real check):
   1. Click a folder in the tree. Arrow / PageUp / PageDown should still move the tree.
   2. Click a tile. Arrow / Page / scroll keys should move the grid. (Unchanged.)
   3. Click a folder again, then click the empty canvas of the tile pane (not a tile, not a section header). Arrow / PageUp / PageDown / scroll keys should now move the grid, not the tree.
   4. Click a section header if one is visible — Left/Right should still expand/collapse as today.
   5. Type in search (or open Tags & notes). Canvas-click handler must not yank focus from those; clicking the canvas after leaving them should focus the grid the same way a tile click does.

Notes

• No CI/signing rewrite, visibility unchanged.
• After merge, auto-tag should create v0.8.3-beta1.