Migrate FileViewer header to PageHeader component

[dimakis]

Context

Part of the mobile UI overhaul (PR #190). FileViewer was skipped during PageHeader adoption because it has complex back-navigation logic (dirty checks, fromRoute state).

Task

• Replace the custom header in FileViewer.tsx with the shared PageHeader component
• Preserve the existing fromRoute/dirty-check logic in the onBack handler
• Update related tests

Related

• PR feat(ui): mobile UI overhaul — tab bar, design tokens, layout fixes #190 (mobile UI overhaul)