feat(i18n): per-session i18n routing and localized scenarios

Phase C of #188 — the last phase of server-side multi-language support.

Adds Session.I18n(), a cached per-session i18n table resolver built via
getLangStringsFor(s.Lang()). The pointer is stable until SetLang
invalidates the cache, so hot-path handlers pay zero allocations on
repeated calls. All 51 s.server.i18n.* call sites across commands,
guild, guild scout, cafe, and cast-binary handlers now route through
s.I18n().*, so chat replies, guild invite mail templates, cafe reset
notices, and quest-timer broadcasts are served in the player's
preferred language instead of the server-wide default.

Scenario JSON gets the same plain-or-map LocalizedString treatment
that quests received in phase B: subheader Strings and inline entry
Text accept either a plain string (backwards compatible) or a
language-keyed object. CompileScenarioJSON takes the compiling
session's language, loadScenarioBinary passes s.Lang(), and
ParseScenarioBinary emits plain-string LocalizedStrings so existing
.bin files round-trip byte-for-byte through the JSON path.

World-wide broadcasts (Raviente siege announcements via
BroadcastRaviente) intentionally stay on the server default — they
have no single-session context to resolve against.

server/channelserver/handlers_cast_binary.go

@@ -49,7 +49,7 @@ func handleMsgSysCastBinary(s *Session, p mhfpacket.MHFPacket) {
 				_ = tmp.ReadBytes(9)
 				tmp.SetLE()
 				frame := tmp.ReadUint32()
-				sendServerChatMessage(s, fmt.Sprintf(s.server.i18n.timer, frame/30/60/60, frame/30/60, frame/30%60, int(math.Round(float64(frame%30*100)/3)), frame))
+				sendServerChatMessage(s, fmt.Sprintf(s.I18n().timer, frame/30/60/60, frame/30/60, frame/30%60, int(math.Round(float64(frame%30*100)/3)), frame))
 			}
 		}
 	}