multiple UX refinements, PWA enablement, and docs

[markrai]

• Board (mobile) — When a todo drag starts, lane tabs briefly flash (300ms) so it is obvious they accept drops; tab labels stay readable above the drop overlays.
• Web Push (PWA) — After sign-in, the client auto-subscribes when both VAPID keys are set on the server; SCRUMBOY_PUSH_BY_DEFAULT_IF_VAPID removed (VAPID presence is the operator signal). Per-user autosub progress in localStorage with resilient retry when the permission prompt is dismissed vs blocked.

Fixes

• Board (drag-and-drop) — Success toast “Todo moved to …” only when the todo changes lane; same-lane reorder no longer shows a redundant toast (lane titles still come from the board workflow, not hardcoded names).

Improvements

• Settings → Customization — Background notifications (PWA) is grayed out with a one-line notice when Web Push is unavailable (no VAPID on the server, or anonymous board mode).

Documentation

• docs/mcp.md — MCP documentation added/expanded.
• docs/pwa.md — Push flow and env vars aligned with streamlined enablement; key generation note includes **