fix: register ini codec for config loading

[wcollins]

Description

Unblocks #193. Viper 1.20 removed built-in INI support, and INI is the default config format for ipctl, so the Dependabot bump breaks all config loading (decoder not found for this format) and hangs TestConfigLoadingIsThreadSafe until the CI test timeout. This PR upgrades Viper to 1.21.0 and restores INI support by registering the codec Viper extracted (github.com/go-viper/encoding/ini).

Type of Change

• Bug fix

Changes Made

• Bump spf13/viper 1.19.0 -> 1.21.0 and vendor go-viper/encoding/ini
• Register the INI codec via viper.NewCodecRegistry() in Loader.Load(); the registry falls back to built-in YAML, JSON, and TOML codecs for other formats
• Fix TestConfigLoadingIsThreadSafe to count failed goroutines as completed, so config errors fail in seconds instead of deadlocking CI
• Regenerate NOTICE for the changed dependency tree (drops hcl, magiconair/properties, slog-shim; adds go-viper packages, locafero, go.yaml.in/yaml/v3)

Testing

• make test passes; internal/config completes in 0.3s (previously hung on Viper 1.21)
• Verified with the built binary: INI configs load via --config, extensionless ~/.platform.d/config, and config.ini discovery; YAML configs still load through the codec-registry fallback; malformed INI returns a clear parse error
• go-licenses report passes

Checklist

• Code follows the project's style guidelines
• Self-review of code has been performed
• Commits follow conventional format
• No secrets or credentials committed