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_cafe.go

@@ -103,7 +103,7 @@ func handleMsgMhfGetCafeDuration(s *Session, p mhfpacket.MHFPacket) {
 	bf.WriteUint32(uint32(cafeTime))
 	if s.server.erupeConfig.RealClientMode >= cfg.ZZ {
 		bf.WriteUint16(0)
-		ps.Uint16(bf, fmt.Sprintf(s.server.i18n.cafe.reset, int(cafeReset.Month()), cafeReset.Day()), true)
+		ps.Uint16(bf, fmt.Sprintf(s.I18n().cafe.reset, int(cafeReset.Month()), cafeReset.Day()), true)
 	}
 	doAckBufSucceed(s, pkt.AckHandle, bf.Data())
 }