feat: use photo set selector in add-photos-to-project window

[siculo]

Summary

Extend the add-photos-to-project window to use the shared photo set
selector introduced in issue #126. Currently the window always adds
the photos selected in Lightroom; it should also allow adding the
photos currently visible in the grid.

Motivation

A user browsing a filtered view in Lightroom (e.g. photos from a
specific day or location) should be able to add the entire visible
set to a project without having to manually select all of them first.
Using the same selector component as the other flows keeps the
interaction consistent.

Scope

Plugin

• Add the photo set selector (radio buttons or popup menu) to the
  add-photos-to-project window, with the options:
  • Selected photos (default — preserves current behaviour)
  • Visible photos
• The photo count shown in the window updates when the selection
  changes
• Reuse the component implemented in issue feat: add photo set selector (selected / visible) to remove photos and activity creation #126; no new selector
  logic needed

API

• No changes required

Dependencies

• Depends on issue feat: add photo set selector (selected / visible) to remove photos and activity creation #126 (shared photo set selector component)

Acceptance criteria

• Add-photos window shows the photo set selector defaulting to
  "Selected photos"
• Choosing "Visible photos" adds all photos currently shown in
  the Lightroom grid to the project
• Photo count in the window reflects the chosen set
• No regression in the existing add-photos flow