Remove dead CSS from pre-PageHeader era

[dimakis]

Context

After PR #190 adopted PageHeader across all pages, several old per-page header CSS classes are now unused.

Dead classes to remove

• .inbox-header, .inbox-back
• .todo-header, .todo-back
• .task-board-header, .task-board-back
• .cal-header, .cal-back
• Any other orphaned header/back button styles from before PageHeader

Task

• Grep for usage of these classes in TSX files to confirm they're dead
• Remove from global.css
• Verify tests still pass

Related

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