feat(ui): add slash-key search and keyboard navigation to application details dropdown

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Summary

• Replace the old DropDown-based app switcher in the application details page with a custom portal-rendered dropdown that supports pressing / to open search, arrow keys to navigate, Enter to select, and Escape to close.
• Add keyboard-highlighted rows with scroll-into-view behavior for long app lists.
• Render the dropdown panel via ReactDOM.createPortal into document.body so it escapes any ancestor overflow: hidden containers.
• Add unit tests for the dropdown component (open/close, service call, panel rendering).

Closes #27904

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argo-cd-ui--lightmode-slash-filter.mp4

Checklist

• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• [n/a] Does this PR require documentation updates?
• [n/a] I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.

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This PR was created with AI-assisted tooling. All code has been manually reviewed and tested.

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