DRU-390 - Issue board, list, and ticket pages, mounted from Software Factory

[chaosk]

Linear ticket: DRU-390

Stacked on DRU-389. Canonical 390 (draft #412 is the superseded first attempt).

Summary

• Python pages at /board, /list, and /tickets/{identifier}. Board is not / — Overview already owns that.
• SPA ui.tsx adds Board and List tabs and mounts AppPage. App.navigation stays [].
• list_pages_for_app allows zero / landings (still fails on two).

Test plan

• uv run ruff check backend
• uv run ruff format --check backend
• uv pip install -e backend/tests/druks-field_notes
• uv run pytest backend/
• npm --prefix frontend run lint
• npm --prefix frontend test
• Empty install shows Overview plus Board and List tabs; New ticket / New project live on /software_factory/board.
• Creating a ticket from the board lands it in Todo on board and list.
• Cancelled tickets are absent from the board and present at the bottom of the list.