feat: add photo set selector (selected / visible) to remove photos and activity creation

[siculo]

Summary

Add a reusable photo set selector to any plugin operation that works
on a subset of photos. The selector lets the user choose between
selected photos and visible photos (those currently shown in
the Lightroom grid). The same UI component is used in both the
remove-photos flow and the activity creation modal.

Motivation

Currently, remove photos always acts on the selected photos and
activity creation offers only "all project photos" or a manual
selection. Making the photo set explicit and consistent across
operations reduces surprises and gives the user direct control
without leaving the plugin dialog.

Scope

Plugin — shared photo set selector component

Implement a reusable UI component (radio buttons or popup menu)
with the following options, available wherever a photo subset must
be chosen:

Option	Source	SDK method
Selected photos	Photos selected in Lightroom	catalog:getMultipleSelectedOrAllPhotos()
Visible photos	All photos currently shown in the grid	catalog:getTargetPhotos() (no active selection)

The component must be callable from any dialog that needs it.

Plugin — remove photos

• Add the photo set selector to the remove-photos confirmation dialog
• Default: Selected photos
• The confirmation message updates to reflect the chosen set and
  the resulting count

Plugin — activity creation modal

• Extend the existing photo subset selector with two new options:
  Selected photos and Visible photos, in addition to the
  existing All project photos
• Default: All project photos (preserves current behaviour)
• The summary panel updates the photo count when the selection changes

API

• No changes required; both flows already send an explicit
  photo_ids list (or null for all photos)

Documentation

• Update the plugin user guide

Acceptance criteria

• Remove-photos dialog shows the photo set selector defaulting
  to "Selected photos"
• Activity creation modal shows all three options; "All project
  photos" remains the default
• Selecting "Visible photos" uses catalog:getTargetPhotos()
  to build the photo list
• Selecting "Selected photos" uses
  catalog:getMultipleSelectedOrAllPhotos() to build the list
• Photo count in confirmation/summary reflects the chosen set
• The selector component is implemented once and reused in both
  dialogs
• No regression in existing remove-photos or activity creation
  behaviour