FR: Add Generate Task to Galleries

[Gykes]

This PR adds a "Generate" option to the gallery detail page overflow menu, addressing the issue where users had no way to regenerate broken image thumbnails for specific galleries. When a user clicks "Generate" from a gallery's overflow menu, the system queries for all image IDs within that gallery and opens the Generate dialog with those images pre-selected.

Tested by deleting my image previews and then regenerating with this feature

Dropdown:

Modal:

Fixes: #5756

[DogmaDragon]

One improvement outside the initial feature request for consistency across objects would be to add the Generate option to the Galleries page. Which would allow selecting multiple galleries and running the generate task more selectively in bulk.

Scenes can trigger Generate... from Scenes page overflow menu and Generate from the individual scene page via overflow menu.

I can create an issue if it's out of scope for this PR.

[Gykes]

I don't have an issue with it but will need to figure out how it works. Currently we pass the ID for the img/scene for generation. I think we would need to parse down one extra level with galleries. Definitely doable but lets make sure my current implementation is even correct before I go down that rabbit hole.

I would make a issue and if WP signs off on my current work then happy to look at it :)

[sleetx]

UI tip: A menu option that triggers a popup needing user interaction should have ellipsis ("...") to indicate that. If the menu option will trigger an action with no intermediary popup, then go without ellipsis.
https://learn.microsoft.com/en-us/windows/win32/uxguide/cmd-menus#using-ellipses

[discourse-stashapp]

This pull request has been mentioned on Stash. There might be relevant details there:

https://discourse.stashapp.cc/t/stash-v0-31-release-development-thread/4881/1