Update and align selection indicator styles across the UI

## Overview

We currently have inconsistent selection indicator patterns across the application. This issue tracks the effort to audit and align these styles.

## Current patterns in use

- Blue highlight background for selected items
- Checkmarks (sometimes before text, sometimes after)
- Radio buttons (primarily on admin pages)
- Grey highlight background

## Proposed direction

- Remove blue highlight backgrounds in favor of a more subtle approach
- Standardize on checkmarks as the primary selection indicator
- Use grey backgrounds only where needed for visual clarity

## Design reference

[Figma component](https://www.figma.com/design/WfqIgsTFXN2BscBSSyXWF8/Coder-kit?node-id=550-618&t=s96BGc1OcTwqsjcp-1)

## Tasks

- [ ] Audit all selection patterns currently in use across the application
- [ ] Align with the Figma component design
- [ ] Update components to use consistent selection indicators
- [ ] Remove unnecessary blue highlight backgrounds

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Created on behalf of @chrifro