127 create shared layout for all pages

[wilhelsr]

Purpose

Create a shared layout to give all pages consistent width and padding, and remove duplicated page-level layout styles.

Changes

• Add a shared Layout component containing navigation, main content, and footer.
• Centralize content width and horizontal and vertical padding.
• Update all pages to fill the layout’s available width.
• Remove page-level minimum heights so the footer stays at the bottom on short pages.
• Keep notifications and the back-to-top button in the shared layout.

Related issues

Closes #127

Verification

Run from frontend/:

• npm run build — passed after the padding changes.
• npm run lint — passed during the layout implementation.
• npm test — all 106 tests passed during the layout implementation.

Full manual testing across viewport sizes has not been completed.

Screenshots

Not applicable.

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have included screenshots for visual changes, or marked them not applicable.

[krgjert]

Good, now all pages uses same padding and style!