Export Presets main button runs selected frames, preview frame when only one selected

[jboneng]

Summary

Changes the Export Presets main button from “current frame only” to selection-aware export: multi-selected filmstrip frames export through every enabled preset; a single (or empty) selection exports the frame shown in the preview.

The chevron menu is unchanged: Export current frame and Export all visible frames… (with confirmation).

Changes

Export Presets scope (main button)

• 2+ frames selected → export all selected frames × enabled presets (filmstrip display order, selected frames hidden by filter still included)
• 0–1 selected → export the preview frame (selected_file_idx / state.config), so keyboard navigation through a multi-select doesn’t surprise you
• No confirmation for the main button (confirmation remains on Export all visible frames… only)

Refactor

• Shared preset-export path: _preset_export_files_for_selection(), _build_preset_export_tasks(), _dispatch_preset_export()
• request_preset_export() (menu: current frame) and request_preset_batch_export() (menu: all visible) unchanged in intent

UI copy

• Tooltip and tutorial step updated to describe selected-frame default + menu alternatives

Test plan

• One frame selected / normal single-frame workflow → Export Presets exports preview frame through all enabled presets
• Multi-select 2+ thumbs → Export Presets exports each selected frame (verify output count = frames × presets)
• Multi-select + arrow to another preview with one thumb highlighted → exports preview frame, not stale selection
• Menu → Export current frame still exports active frame only
• Menu → Export all visible frames… still shows confirmation and exports all visible
• Excluded frames in a multi-selection are skipped
• uv run pytest tests/test_controller.py::TestPresetExportSelected tests/test_controller.py::TestPresetBatchExport -v