Add export-all-visible option to Export Presets with confirmation dialog

[jboneng]

Summary

Export Presets previously ran only on the current frame. This PR adds a batch path that exports every visible filmstrip frame through every enabled preset, with a confirmation step before large jobs start.

The Export panel Presets row is also reworked into a split control: main click = current frame; chevron menu = scope choice (current frame or all visible).

Changes

Export presets — batch scope

• request_preset_batch_export() walks visible filmstrip indices (respects sort + filter, same as Export All / Contact Sheet).
• Each frame uses its own saved edit from the DB; each enabled preset runs through the existing _tasks_for_file() / resolve_preset_export() pipeline (Print vs Flat per preset unchanged).
• Confirmation dialog before batch runs, e.g. “Export 36 frames through 3 presets (108 files)?” — Cancel aborts with no export.

UI — split Export Presets control

• Main button — one-click export of the current frame through all enabled presets (unchanged behavior).
• Chevron menu — explicit choices:
  • Export current frame
  • Export all visible frames…
• Manage button stays compact; Export Presets expands to fill the row.
• Styled as a unified split button in modern_dark.qss (centered label, larger chevron, no duplicate Qt menu arrow).

Docs & tests

• Tutorial export step updated to mention the split button.
• TestPresetBatchExport — filter scope, cancel, empty visible set, confirmation message.

Test plan

• Enable 2+ presets; click Export Presets main area → only current frame exports (N files for N presets).
• Open chevron menu → Export current frame → same as above.
• Open chevron menu → Export all visible frames… → confirmation shows correct frame × preset × file counts.
• Confirm Cancel on dialog → no export starts.
• Confirm Yes → all visible frames export through each enabled preset, hidden/filtered frames skipped.
• Filter filmstrip to a subset → batch export only includes visible frames.
• Flat + Print presets mixed → each preset’s render intent respected per frame.
• uv run pytest tests/test_controller.py::TestPresetBatchExport -v
• uv run ruff check