feat: start boards from templates, vote with dots and print boards

[AmrMsCLL]

Boards step B4.

• Templates: an empty board (once synced, so a loading board never shows it) offers Kanban, Retro, Mind map, Flowchart, Weekly planner and Mood board. Picking one fills the board through Excalidraw's convertToExcalidrawElements and zooms to it; the arrows in the mind map and flowchart are bound to their shapes. Templates live in components/board-templates.ts.
• Voting dots: Add a vote drops a dot in your colour near the middle of the view.
• PDF: the board menu gains Print or save as PDF, which prints a PNG of the board from a hidden frame (the browser's print dialog saves PDFs).

Checked in the browser (local stack): a new board showed the templates, Kanban filled it (To do, Doing, Done and a first card) and the picker went away, and Add a vote added a dot in my colour; both reached the server. Not checked: the print dialog.

Tests: new test/board-templates.spec.tsx, lint, type check.