Fix mixed theme state after sign-in route transition

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Summary

• Fixes intermittent mixed color theme state after sign-in where UI could render with conflicting light/dark tokens until refresh.
• Hardens Harmony ThemeProvider document theme synchronization for nested providers.

Root Cause

• Both the global app shell and sign-on flow mount Harmony theme providers.
• Each provider mutates html[data-theme]; during auth route transitions/unmount timing, the document theme could be left in a stale state.

Changes

• Updated packages/harmony/src/foundations/theme/ThemeProvider.tsx to track mounted provider instances and always apply the active top-most provider theme to html[data-theme] on mount/update/unmount.
• Ensures unmounting an inner provider restores the parent provider theme deterministically.

Validation

• Attempted targeted typecheck with npm run -w @audius/harmony typecheck.
• In this automation environment, dependencies are not installed (tsc: not found), so full local validation could not be completed here.

  

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