Fix volume asset dropdown not populated and crash when no volume selected in Match Template panel

[Copilot]

This PR fixes two related issues in the Match Template panel:

1. Volume assets not appearing in dropdown when switching workflows
2. Application crash when attempting to run with no volume selected

Root Cause

The primary issue was in the DirtyVolumes() and DirtyImageGroups() methods in MainFrame.cpp, which only marked template matching panels as dirty when the current workflow was "Template Matching":

if ( current_workflow == "Template Matching" ) {
    match_template_panel->volumes_are_dirty = true;
}

This created a scenario where:

1. User opens a project while in "Single Particle Analysis" workflow
2. Volume and image group dropdowns for Match Template panel are never populated
3. User switches to "Template Matching" workflow
4. Dropdowns remain empty because they were never marked dirty
5. User clicks "Run" without being able to select a volume → crash

Stack Trace Analysis

The crash occurred because ReferenceSelectPanel->GetSelection() returns -1 when no item is selected, which gets passed to volume_asset_panel->ReturnAssetPointer(-1), triggering a debug assert:

Requesting an asset (-1) that doesn't exist!
Failed Assert at ../../../src/core/assets.cpp:1071
VolumeAsset *VolumeAssetList::ReturnVolumeAssetPointer(long)

Solution

1. Fixed workflow-dependent dirty flag setting:

• Modified DirtyVolumes() and DirtyImageGroups() to always mark template matching panels as dirty, regardless of current workflow
• This ensures dropdowns are properly populated when users switch to Template Matching workflow

2. Added validation to prevent crashes:

• Added volume selection check in OnUpdateUI() to disable run button when no volume is selected
• Added safety check in StartEstimationClick() with user-friendly error message

Changes Made

• src/gui/MainFrame.cpp: Fixed workflow-dependent dirty flag setting
• src/gui/MatchTemplatePanel.cpp: Added volume selection validation and safety checks

All changes are minimal and surgical, addressing only the specific issues without affecting existing functionality.

Fixes #40.

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