fix: keep footer at the bottom of the viewport on short pages

[nyg]

Problem

When a view has little content (for example the Issues tab with filters that match nothing), the footer sits right under the content and leaves an empty band below it instead of reaching the bottom of the window.

.app-footer already has margin-top: auto, but that only pushes an element down inside a flex container. Its parent, #root, was a plain block, so the rule had no effect.

Fix

Make #root a full-height flex column in src/styles/base.css:

#root { display: flex; flex-direction: column; min-height: 100vh; }

The footer's existing margin-top: auto now pushes it to the bottom of the viewport on short pages. Long pages scroll as before, with the footer after the content. The sticky top bar and sidebar, board focus mode, and the auth screen are unaffected.

Testing

• Checked in the running app on the Issues tab with a search that matches nothing, at 1600×1000: before the change the footer's bottom edge was at 556px; after, it is at 1000px.
• npm run build passes.

Screenshots

Before

After