Add batch selection actions for gallery lists

[Vivitoto]

Summary

This PR adds an opt-in batch selection mode to gallery lists.

It supports:

• selecting/deselecting loaded gallery items
• selecting all currently loaded items
• batch favorite with one favorite slot selection
• batch unfavorite with confirmation
• copying torrent magnet links for selected galleries

When multiple torrents are available for one gallery, the selected torrent can be chosen by:

• largest size
• latest time

The chosen magnet links are copied to the clipboard as newline-separated text.

Behavior

Existing gallery item behavior is preserved:

• normal tap still opens/navigates to the gallery
• normal long press still opens the existing single-gallery action menu

Batch mode is only enabled explicitly from:

• the top bar batch selection action
• the existing single-gallery action menu

In batch mode, tapping a gallery toggles its selected state.

UI

Batch mode uses the existing selectable item overlay style.

For gallery lists, the selection indicator reserves a stable top-right area while batch mode is active. Unselected items show an outline indicator, and selected items show a checked indicator. This avoids layout shifts and keeps long titles from colliding with the selected state indicator.

Notes

"Select all" only selects currently loaded paging items.

Batch favorite/unfavorite operations are processed sequentially to avoid firing many requests at once.

Test plan

Passed on fork CI:

• check
• default
• marshmallow

CI run:
https://github.com/Vivitoto/EhViewer/actions/runs/34434538759

A pre-release APK build for manual testing is available here:
https://github.com/Vivitoto/EhViewer/releases/tag/batch-gallery-actions-3d07b1e

[revonateB0T]

@Vivitoto Hi! Thanks for the PR, could you please split the torrent-related logic into a separate PR so we can review it first? The GalleryListScreen UI needs to be discussed, if we decided to add multi-selection, the long-press dialog would become unnecessary, should consider moving the "Read" button somewhere else.

[Vivitoto]

Thanks for the feedback. I split the torrent-related refactor into a separate smaller PR first: #3009.

For this PR, I will keep the gallery-list multi-selection UI discussion separate and will not add more torrent behavior here.