feat(mobile): Show linked pull requests in the thread header#89

What Changed

The mobile thread screen shows a pull request button in its header when the thread has linked pull requests or a pull request for its current branch, matching the desktop indicator. The glyph and tint follow the PR state (open, draft, merged, closed, not yet synced), and stacks and multiple linked PRs show a layer or +N count with their aggregate state. A second glyph appears only when something needs attention: failing checks or a merge conflict. Passing and running checks show no glyph and stay in the accessibility label. Every state differs in shape or text as well as color, so it reads on grayscale screens. Tapping opens the PR in the app's PR screen; with several linked PRs it opens a native menu listing each one with its state and checks.

The button sits just left of the rightmost header control. On iOS it joins the Terminal, Files and Git glass group (iPhone: Terminal, Files, PR, Git). On Android it sits left of the inspector toggle, or of the overflow button or last action when the toggle is absent or folded into overflow. Narrow panes, including phones and narrow split panes, use a compact form without the #N text so the thread title keeps its room; the number stays in the accessibility label, the PR screen and the menu. Panes at least 600 points wide show the full form. On Android the header budgets its width and moves direct actions, then the inspector toggle, into the overflow menu when they do not fit. The Android menu adapter now forwards accessibility props to its trigger.

Why

Desktop already shows a thread's linked PRs at a glance; mobile only showed them in the thread list. This is a fork-only layer: upstream did not want it.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes