fix(plugin store): plugin store closes on installing

[AdityaKr015]

Summary

This PR fix the issue where installing plugin closes the store rather than keep it open. Swaps Add button to spinner on installing.

Motivation

Closing the store on every install forces users to reopen it and scroll back for plugin again, keeping it open makes multiple installs painless.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Closes #4055

Testing

• Ran just format, just build, just test everything went smoothly
• Killed noctalia and starts build-debug
• Tested with installing plugin and store stay open.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

recording_20260823_192922.mp4

Checklist

• This PR is ready for review.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed.
• I ran the relevant build or test commands.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.