Description

Currently no panel is focusable — Tab does nothing and all panels render identically. Add keyboard focus cycling so users can select a panel to interact with (scroll, navigate items, zoom).

Implementation:

• DashPanel already has :focus CSS (border: round $accent). Need to make panels focusable: set can_focus = True on DashPanel
• Add Tab / Shift+Tab bindings that cycle focus through panels on the active page
• Focused panel gets the accent border; unfocused panels use the standard panel border
• Focus state resets when switching pages (first panel on new page gets focus, or no focus)
• Scroll within a focused panel works naturally since DashPanel extends ScrollableContainer

This is a prerequisite for: keyboard-navigable items (TASK-007), in-panel search (TASK-009), panel zoom (TASK-006), quick open link (TASK-008).

Acceptance Criteria

• #1 Tab cycles focus forward through panels on the active page
• #2 Shift+Tab cycles backward
• #3 Focused panel has accent-colored border
• #4 Switching pages resets focus
• #5 Existing scroll behavior within panels is unaffected