[codex] Fix terminal custom accent color

[binaricat]

Summary

• Make terminal cursor, selection, active terminal toolbar buttons, and terminal side-panel active buttons follow the custom accent color.
• Keep theme-based terminal colors unchanged when custom accent is disabled or invalid.
• Add regression coverage for terminal accent mapping and pressed toolbar state.

Root cause

Terminal pages apply immersive terminal theme colors. That path used the terminal theme cursor color as the app accent, so a custom appearance accent was overwritten back to the terminal theme default blue.

Validation

• npm run lint
• npm test (311 passing)
• npm run build

Closes #860