feat(config): register all defaults in code, shrink example config

Only the database password is truly mandatory to get started, but
config.example.json was 267 lines with 90+ options. Newcomers faced a
wall of settings with no indication of what matters.

Add registerDefaults() with all sane defaults via Viper so a minimal
config (just DB credentials) produces a fully working server. Gameplay
multipliers default to 1.0 instead of Go's zero value 0.0, which
previously zeroed out all quest rewards for minimal configs. Uses
dot-notation defaults for GameplayOptions/DebugOptions so users can
override individual fields without losing other defaults.

- Shrink config.example.json from 267 to 10 lines
- Rename full original to config.reference.json as documentation
- Simplify wizard buildDefaultConfig() from ~220 to ~12 lines
- Fix latent bug: SaveDumps default used wrong key DevModeOptions
- Add tests: minimal config, backward compat, single-field override
- Update release workflow, README, CONTRIBUTING, docker/README

server/channelserver/testhelpers_db.go

@@ -185,7 +185,7 @@ func CreateTestCharacter(t *testing.T, db *sqlx.DB, userID uint32, name string)
 	// Create minimal valid savedata (needs to be large enough for the game to parse)
 	// The name is at offset 88, and various game mode pointers extend up to ~147KB for ZZ mode
 	// We need at least 150KB to accommodate all possible pointer offsets
-	saveData := make([]byte, 150000) // Large enough for all game modes
+	saveData := make([]byte, 150000)                // Large enough for all game modes
 	copy(saveData[88:], append([]byte(name), 0x00)) // Name at offset 88 with null terminator
 
 	// Import the nullcomp package for compression