Derive button placement paths and name-to-feature mappings from metadata registry

[VampireChicken12]

Summary

The buttonNameToSettingName lookup object, buttonNames array, featureToMultiButtonsMap, and hardcoded switch statements in ButtonPlacement.tsx/ButtonPlacementSetting.tsx all duplicate knowledge that now exists declaratively in feature metadata (button field).

Changes Needed

1. Add button maps (buttonNameToFeature, featureToButtonNames) to FeatureMetadataRegistry
2. Add getter methods: getButtonFeature(), getAllButtonNames(), getButtonNamesForFeature(), getButtonConfigPath()
3. Replace @/src/types const imports with registry calls in 5 files (ButtonPlacement.tsx, ButtonPlacementSetting.tsx, utils/settings/index.ts, features/buttonPlacement/index.ts, utils/feature/index.ts)
4. Eliminate hardcoded switch statements in path-derivation functions

Status

Implementation is complete — 22 files are ready to commit on the dev branch.