fix(ui): decode room id from URL and encode it when writing back

[LandmineHQ]

fix(ui): decode the room id from the URL and encode it when writing back

Fixes #253

Problem

ui/src/useRoomID.ts reads and writes the room query parameter by hand — no decode on read, no
encode on write:

search.slice(1).split('&').find((param) => param.startsWith(`${key}=`))?.split('=')[1]
window.history.pushState({roomId: id}, '', id ? `?room=${id}` : '?');

Browsers percent-encode non-ASCII names when serializing a URL, so a room named 不辞 is shared as
?room=%E4%B8%8D%E8%BE%9E, but the client then joins the literal %E4%B8%8D%E8%BE%9E room and the
server closes the connection with room with id %E4%B8%8D%E8%BE%9E does not exist. Ids containing
&, =, #, + or % break in the same way, and writing the escaped value back adds another
escaping level on every share.

Changes

Only ui/src/useRoomID.ts: the query string now goes through URLSearchParams, which decodes once
on read and encodes once on write.

-export const getFromURL = (key: string, search = window.location.search): string | undefined =>
-    search
-        .slice(1)
-        .split('&')
-        .find((param) => param.startsWith(`${key}=`))
-        ?.split('=')[1];
+export const getFromURL = (key: string, search = window.location.search): string | undefined =>
+    new URLSearchParams(search).get(key) ?? undefined;

                     if (oldId !== id) {
-                        window.history.pushState({roomId: id}, '', id ? `?room=${id}` : '?');
+                        const params = new URLSearchParams();
+                        if (id) {
+                            params.set('room', id);
+                        }
+                        const query = params.toString();
+                        window.history.pushState({roomId: id}, '', query ? `?${query}` : '?');
                     }

No server change: the room id stays the raw string used by the create/join payload, and existing
?room=%E4%B8%8D%E8%BE%9E links work again without migration.

Verification

• cd ui && yarn build (tsc + vite) passes
• prettier --check src/useRoomID.ts passes
• Browser: 不辞 ⇄ ?room=%E4%B8%8D%E8%BE%9E round-trips stably (repeated writes do not re-escape);
  my room, a+b, a&b=c and 100% round-trip correctly as well
• Live 1.12.5 server: joining with the encoded id fails (room with id %E4%B8%8D%E8%BE%9E ... does not exist), joining with the decoded id succeeds, and the test room is removed after its owner leaves

[jmattheis]

Thanks!