[Feature]: Open linked workspace files from mobile Markdown previews

[saphid]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case.

Area

apps/mobile file viewer.

Problem or use case

The mobile file viewer added in #3155 renders Markdown files, but tapping a relative workspace link inside that preview is handled as an external URL. A README or documentation index therefore cannot navigate to another file in the same workspace.

Proposed solution

Classify Markdown links with the existing mobile link parser. Resolve workspace-file links relative to the containing Markdown file, keep navigation inside the current thread file viewer, preserve optional line targets, and continue opening ordinary web links externally. Reject paths that escape the workspace root.

Why this matters

Repository documentation commonly links between README, guide, and source files. In-app navigation makes the existing file viewer useful for following that documentation on mobile.

Smallest useful scope

Support file links from Markdown preview mode only. Reuse the existing ThreadFile route and link presentation helper. Add focused path-resolution tests, including relative links, absolute in-workspace links, and workspace escapes.

Alternatives considered

Opening every link externally does not work for local files. Adding a new browser or navigation surface is unnecessary because the existing file route already owns this behavior.

Risks or tradeoffs

Cross-platform path normalization and traversal protection must remain explicit. The patch should not change web-link handling or the broader file browser.

Contribution

• I would be open to helping implement this.