feat(shutdown): passive drain and rename DisableSoftCrash

Shutdown now proceeds in three phases: close listeners immediately on
signal, broadcast the in-game countdown, wait up to ShutdownDrainSeconds
(default 30) for sessions to disconnect naturally via DrainPassive, then
force-close any stragglers. This prevents players from entering new
quests after the countdown starts, and lets mid-quest sessions finish
saving without being killed mid-write. A second SIGINT during passive
drain cancels it so the force-close phase runs immediately.

Renamed DisableSoftCrash -> DisableShutdownCountdown since the flag
controls the countdown, not crash behaviour. Existing config.json files
keep working via a Viper alias on the legacy key.

Closes #179.

config/config_load_test.go

@@ -142,26 +142,26 @@ func TestLoadConfigDefaultModeWhenInvalid(t *testing.T) {
 // TestConfigStruct tests Config structure creation with all fields
 func TestConfigStruct(t *testing.T) {
 	cfg := &Config{
-		Host:                   "localhost",
-		BinPath:                "/opt/erupe",
-		Language:               "en",
-		DisableSoftCrash:       false,
-		HideLoginNotice:        false,
-		LoginNotices:           []string{"Welcome"},
-		PatchServerManifest:    "http://patch.example.com/manifest",
-		PatchServerFile:        "http://patch.example.com/files",
+		Host:                      "localhost",
+		BinPath:                   "/opt/erupe",
+		Language:                  "en",
+		DisableShutdownCountdown:  false,
+		HideLoginNotice:           false,
+		LoginNotices:              []string{"Welcome"},
+		PatchServerManifest:       "http://patch.example.com/manifest",
+		PatchServerFile:           "http://patch.example.com/files",
 		DeleteOnSaveCorruption:    false,
 		DisableSaveIntegrityCheck: false,
-		ClientMode:             "ZZ",
-		RealClientMode:         ZZ,
-		QuestCacheExpiry:       3600,
-		CommandPrefix:          "!",
-		AutoCreateAccount:      false,
-		LoopDelay:              100,
-		DefaultCourses:         []uint16{1, 2, 3},
-		EarthStatus:            0,
-		EarthID:                0,
-		EarthMonsters:          []int32{100, 101, 102},
+		ClientMode:                "ZZ",
+		RealClientMode:            ZZ,
+		QuestCacheExpiry:          3600,
+		CommandPrefix:             "!",
+		AutoCreateAccount:         false,
+		LoopDelay:                 100,
+		DefaultCourses:            []uint16{1, 2, 3},
+		EarthStatus:               0,
+		EarthID:                   0,
+		EarthMonsters:             []int32{100, 101, 102},
 		SaveDumps: SaveDumpOptions{
 			Enabled:    true,
 			RawEnabled: false,