Add scrollable board picker pages

[devmobasa]

Fixes #218 - Implements proper board picker support for boards with more than 9 pages.

• Adds a scrollable page panel so all pages can be reached from the board picker.
• Keeps page operations on absolute page indexes, so activate/rename/duplicate/delete/reorder work correctly beyond the first visible page window.
• Adds direct page creation after the visible grid is full via a sticky Add button and Ctrl+N.
• Improves large-page navigation with wheel scrolling, arrows, Home/End, PageUp/PageDown, and active/focused page auto-scroll.
• Exposes Ctrl+C: color in the board picker footer and keeps F2 reliably mapped to board rename.