webrtc: fix race edge cases that discard or mislabel a direct connection

Three correctness fixes in the transport race, plus three convention
cleanups.

- race_transports_prefer_webrtc committed a relayed result while a direct
  attempt was still in flight: the others arm returned on
  webrtc_fut.is_none() even with an unfinished direct future, and the
  WebRTC-error arm returned a held relay without checking others_fut. A
  relay is now committed only when nothing direct can still arrive (or
  the window expires); a parked relay is also preferred over composing
  an error when both sides fail. Three regression tests, mutation-checked.

- connect()'s plain select_ok let a TURN-relayed WebRTC win as "first
  success", dropping still-racing UDP/IPv6 direct attempts and reporting
  the relayed pair as direct. It now runs through the same prefer-P2P
  race with each attempt carrying whether its path is direct, and the
  WebRTC future resolves is_relayed() so a TURN win is held behind
  direct attempts, not committed as one.

- The RelayResponse path kept direct == true when a WebRTC win's DTLS
  handshake failed and it fell back to relay, so the relay was reported
  P2P. Clear the flag with the transport switch.

- Trim the OffererGuard doc to the three-line max; move the new
  enable-webrtc localization key to the end of every lang list; the KCP
  option constant moved to hbb_common config::keys (0f663aa).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ExUfAkYbq8UC9pQCiLy8TQ

src/lang/cs.rs

@@ -673,7 +673,6 @@ pub static ref T: std::collections::HashMap<&'static str, &'static str> =
         ("Default trackpad speed", "Výchozí rychlost trackpadu"),
         ("Numeric one-time password", "Číselné jednorázové heslo"),
         ("Enable IPv6 P2P connection", "Povolit připojení IPv6 P2P"),
-        ("Enable WebRTC P2P connection", "Povolit připojení WebRTC P2P"),
         ("Enable UDP hole punching", "Povolit UDP hole punching"),
         ("View camera", "Zobrazit kameru"),
         ("Enable camera", "Povolit kameru"),
@@ -759,5 +758,6 @@ pub static ref T: std::collections::HashMap<&'static str, &'static str> =
         ("Continue", "Pokračovat"),
         ("Browser didn't open? Use the url below to sign in.", "Neotevřel se prohlížeč? Pro přihlášení použijte URL níže."),
         ("Lock canvas", "Zamknout zobrazení"),
+        ("Enable WebRTC P2P connection", "Povolit připojení WebRTC P2P"),
     ].iter().cloned().collect();
 }