Update header and navbar design

Implement the updated **header and navbar** design for v2.0 of the UI.

No files, tests, or design mocks are listed in the issue. Start by locating the shared UI layout/templates and stylesheet files where the header and navbar are defined, then trace any page-specific overrides for those elements. Run the app locally and inspect representative pages in a browser to confirm where the current header/navbar is rendered. Mark it done when the v2.0 header/navbar appears applied consistently across the UI and navigation behavior remains intact.

Scritto dal modello di indicizzazione a partire dal testo della issue.