Rebuild the web UI on Bootstrap 5 with the dark felt theme

The lobby was hand-rolled CSS and looked plain. Vendor Bootstrap 5.3
(MIT) under web/vendor so builds stay offline, load it in dark mode and
bridge the felt/gold palette onto Bootstrap's CSS variables in a new
theme.css; style.css now only owns the game table, cards and scoring
summary.

Rebuild the lobby, waiting room and shared chrome with Bootstrap
components: a shared navbar, creation form as cards with schema-driven
controls, roster as a list group with seat/team badges, a seats progress
bar and a copy-to-clipboard button on the join code. History,
leaderboard, connection/error states and the 404 page use cards, tables
and buttons too; the play table itself is unchanged.

The copy feedback timer is cancelled when the waiting room is rebuilt by
another player joining, fixing a disposed-signal panic that froze the
page mid-match.

README.md

@@ -10,7 +10,10 @@ scientifico** — the four-player, fixed-partnership Italian card game:
   and the `tavolo-app` composition root wiring them together.
   See [server/README.md](server/README.md).
 - **`web/`** — frontend: Rust + [Sycamore](https://sycamore.dev) compiled to
-  WebAssembly, built with [Trunk](https://trunkrs.dev). Card images are the
+  WebAssembly, built with [Trunk](https://trunkrs.dev) and styled with
+  [Bootstrap](https://getbootstrap.com) 5.3 (MIT, vendored under
+  `web/vendor/` so builds work offline) plus the custom felt theme in
+  `web/theme.css`. Card images are the
   CC0 *woodcut napoletane* deck traced from a 1902 Naples print
   ([SONDLecT/woodcut-napoletane](https://github.com/SONDLecT/woodcut-napoletane)).