ui: add Key column to homepage flag tables

[smcguinness]

Description

Add a Key column to the homepage Active and Deleted flags tables in browser/flagr-ui/src/components/Flags.vue.

Motivation and Context

Flag keys are commonly used when evaluating flags via API/debug console and when cross-referencing configurations. Showing the key directly on the home tables reduces click-through and speeds up identification.

How Has This Been Tested?

• Ran frontend lint: cd browser/flagr-ui && npm run lint
• Verified local UI recompiles and serves at http://localhost:8080

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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