[TMCU-991] Post-prod cleanup: remove pure black feature flag and provider wiring from mobile

### Description\n\nMirrors Jira [TMCU-991](https://consensyssoftware.atlassian.net/browse/TMCU-991).\n\nPart of epic #1271 / [TMCU-622](https://consensyssoftware.atlassian.net/browse/TMCU-622).\n\n**Jira status:** To Do\n\n### Jira details\n\n## Context\n\nThe mobile app used preview packages () and a feature flag to opt into the provisional pure-black token set. Once absorbs the pure-black values in the released packages, the preview aliases and any provider wiring are dead code.\n\n## Work items\n\n* Remove package aliases from resolutions; restore standard released versions of , , \n* Remove the patch file for the preview package if one exists\n* Remove any mobile-side feature flag gating pure-black rendering (confirm the exact flag name used in mobile)\n* Remove any / wiring added to the mobile theme provider or root component tree\n* Remove the prop from if it was added as part of this work\n\n## Acceptance criteria\n\n* Mobile only depends on released (non-preview) packages\n* No feature flag references to pure-black preview remain in mobile codebase\n* Dark mode on mobile renders with pure-black tokens automatically via the updated \n* All existing mobile tests pass\n\n\n