feat(web): add open in new tab and explicit merge buttons to people thumbnails in merge and reassign pages

[luisdralves]

Description

When merging people, clicking a person immediately marks them for merging. Instead, this PR adds two icon buttons (shown on hover) to allow:

• Opening the person in a new tab to verify before merging
• Explicitly adding/removing from merge selection

This addresses a personal issue when merging people with large year gaps or blurry featured photos, where I feel the need to verify the person's other photos before confidently merging them.

The blue tint on hover was removed and the selected prop was repurposed for controlling whether the plus or minus button appears.

How Has This Been Tested?

• Open a person's page and select "merge people". Hovering a person's face should reveal the 2 action buttons which should behave as described.
• Select a person for merging. Their thumbnail should appear next to the target person and the action buttons should appear on hover, with a minus for deselecting instead of the plus
• Open a person's page, select couple of photos, and hit "fix incorrect match". This interface should behave as above.

Screenshots (if appropriate)

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

Please describe to which degree, if any, an LLM was used in creating this pull request.

Code and project discovery in general, brainstorming, elaborating the PR description.

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 🖥️web. A maintainer will add the required label.

[luisdralves]

Rebased with v2.3.0 and added keyboard navigation and support for touch devices (buttons always visible)

[danieldietzler]

Code-wise this seems fine to me. @alextran1502 your call if you want this UX

[danieldietzler]

I think you could simplify this to hidden hover:visible, right?

[luisdralves]

This would remove the fade in / fade out animation, but fine by me either way. Let me know what you prefer

[danieldietzler]

Yeah I think that's fine, we don't need that