GitHub Mobile: Rebase Stacked PRs [GA]

### Value Prop
You can now rebase an entire stack of pull requests directly from GitHub Mobile, without needing to switch to a desktop browser. Track rebase progress and status in real time from the stack merge section, so you stay in control of your stacked pull request workflows wherever you are.

### Expected Outcome
Developers working with stacked pull requests often hit blockers when branches fall out of date—and resolving them previously required leaving the mobile experience entirely. This release brings full rebase support to GitHub Mobile, helping teams keep their stacks current and their momentum going, even when they're away from their desks.

The issue describes GitHub Mobile support for rebasing stacked pull requests but names no source files, tests, or contribution entry point. Start by locating the GitHub Mobile implementation and existing stack merge or rebase work; done would require the feature to be available in the mobile experience with rebase progress and status visible.

Written by the indexing model from the issue text.