Add multi-selection to ImageGallery

[Copilot]

Adds multi-selection to the ImageGallery. The last clicked item continues to define selectedViewId for the viewer. Multiple images can be selected and removed together as a single undoable operation via right-click context menu or Delete key.

Description

Previously the ImageGallery only supported single-item selection. This adds standard multi-selection UX (Ctrl+Click to toggle, Shift+Click for range) and exposes a "Remove Selected Images" action for the selection.

Features list

• Ctrl+Click toggles an item in/out of the selection
• Shift+Click range-selects from the last clicked item to the target
• Last clicked item still drives selectedViewId (viewer follows it)
• Multi-selected items highlighted with semi-transparent accent (primary selection retains full highlight)
• Right-click → "Remove Selected Images (N)" — enabled when N > 1
• Delete removes all selected images when multiple are selected; falls back to single removal otherwise
• Escape collapses multi-selection back to the current item
• Multi-image removal is a single undoable operation (one undo step)

Implementation remarks

Selection state is tracked in two new sortedModel properties:

• selectedIndex (existing) — the primary/last-clicked item, drives the viewer
• selectedIndices: [] (new) — the full multi-selection set used for removal and highlighting

getObjectAt(filteredIndex) was added to SortFilterDelegateModel to let the gallery collect Python viewpoint attribute objects for all selected indices at removal time.

Multi-image removal is handled by a new removeImages(images) slot in graph.py that wraps individual removeImage() calls inside a single groupedGraphModification, so the entire batch undoes as one step. Nested macros in Qt's undo stack correctly aggregate into the outer macro.

The onCurrentItemChanged guard in grid/list views prevents keyboard navigation from incorrectly resetting selectedIndices by checking sortedModel.selectedIndex !== root.currentIndex before clearing selection state.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.47%. Comparing base (b55440c) to head (444512c).
⚠️ Report is 14 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3054   +/-   ##
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  Coverage    83.47%   83.47%           
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  Files           81       81           
  Lines        10300    10301    +1     
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+ Hits          8598     8599    +1     
  Misses        1702     1702           

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