Add a setting for opening pull requests in Linear or Graphite

[pekala]

Summary

Pull request badges in the sidebar and composer always opened the provider page. This adds a Source control settings page with an "Open pull requests in" choice: GitHub (default), Linear, or Graphite.

• Linear rewrites github.com to linear.review and keeps the rest of the path, query, and fragment.
• Graphite opens https://app.graphite.com/github/pr/{owner}/{repo}/{number}. GitHub sub-pages don't carry over because Graphite has its own page layout.
• Links that are not GitHub pull requests (issues, compare views, other hosts) keep their own page under every choice.
• The target is read from the settings store at click time, so changing it applies to badges already on screen. The badge tooltip adds "Opens in Linear" / "Opens in Graphite" when a rewrite will actually happen.
• Persisted as pullRequestLinkTarget in ui-settings.json (github / linear / graphite). Older settings files without the key load as GitHub.
• Dev route: ZERON_OPEN_ROUTE=settings/source-control.

The iOS app has its own PR badge but no settings screen, so it is unchanged.

Test plan

• cargo test --locked -p zeron-ui --lib -- --test-threads=1 (783 passed). New tests cover the Linear and Graphite rewrites, the non-PR fallbacks for every target, the tooltip hint, and the settings round-trip plus default for older files.
• cargo clippy -p zeron-ui --tests reports nothing in the changed files; changed files are rustfmt-clean.
• Booted the debug app on settings/source-control, switched between the three options, and confirmed the persisted value.
• Click a badge with Graphite selected from an account that has Graphite access, to confirm the app.graphite.com host resolves the /github/pr/… route the same way app.graphite.dev does.

🤖 Generated with Claude Code

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