[nwparker]

Summary

• Adds an "Add repo" action at the bottom of the repo filter dropdown in the sidebar search bar, separated by a divider
• Uses the existing addRepo() store action (same as the toolbar button)

Closes #20

Test plan

• Open the app with 2+ repos so the repo filter dropdown is visible
• Click the repo filter dropdown and verify "Add repo" appears at the bottom with a separator
• Click "Add repo" and verify the folder picker opens
• Verify selecting a normal repo filter still works as expected

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