A fork's repository home says "N commits behind <upstream>" from the last follow check, with its one action to update #5867

The fork tree only works if a fork can see it is falling behind. getFollow (src/core/upstream_follow.ts) already knows how many commits behind a fork is. Today only the upstream-follow page and the terminal's about screen show it.

Done when:

• [ ] On a fork's home (src/web/pages/repo.ts), one line under the head reads 12 commits behind trevato/forge · checked 2 h ago. It links to the upstream-follow page, where updating already lives. A fork with a conflict says can't follow: conflicts instead.
• [ ] At 0 behind, or for a repository that follows nothing, no line is drawn.
• [ ] The read is a named Server-Timing phase under 2 ms. It uses the stored check and never runs git on render.
• [ ] Tests: behind, level, conflicted, and not a fork.