refactor: single-page app — restore master tracking grid, drop /master route

Previous commit misread "remove master board" — restore the 11x9
ones-digit-aligned tracking grid (with circular tokens + draw-order
overlay) inside MasterPanel; remove the host's own player card
("Bảng của quản trò") instead. The host can use the player board
above it; no need for a duplicate card.

Delete /master route entirely (single-page app now). Cloudflare
Pages redirect added via static/_redirects: any unknown path
301-redirects to /, so old /master bookmarks land on the homepage.
Static assets are served first so the rule only fires on misses.

Storage keys loto_master_card_grid / loto_master_card_crossed are
no longer written but old saved data stays in users' localStorage
(harmless leftover).

web/static/_redirects

@@ -0,0 +1,4 @@
+# Cloudflare Pages: any unknown path → 301 redirect to homepage.
+# Static asset files are served first, so this only fires on misses.
+# Keeps old /master bookmarks (and any future stray URL) on the app.
+/* / 301