refactor(config): replace manual viper mapping with Unmarshal (#870)

* refactor(config): replace manual viper mapping with Unmarshal

- Replace 84 manual viper.GetX() calls with viper.Unmarshal() using yaml struct tags
- Remove 116-line defaultConf YAML blob, consolidate all defaults into setDefaults()
- Use viper.New() instead of global singleton to prevent test pollution
- Extract shared test assertions into assertCommonConfig helper
- Remove tests that referenced deleted defaultConf variable
- Fix Huawei test that relied on placeholder default values

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* style(config): merge duplicate Android default blocks

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(config): fail fast on config parse errors and fix misleading comment

- Fix misleading comment about config file search behavior
- Return error on config parse/permission failures instead of silently
  falling back to defaults; only ignore ConfigFileNotFoundError

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: appleboy