Design: a migration's progress stages draw §5's row and badge, and stage-dot/stage-detail leave §5d #4082

Replaces #3872 (parked twice on the design check's baseline test). Waits for #4063 so the check names what to change.

Done:

• [ ] The migrate page's and the browse view's progress stages are drawn from row and badge in src/web/ui.ts, in place of .stage-dot and .stage-detail; src/web/client.ts selects those stages by data-* attributes instead of class names, so the live progress still updates.
• [ ] Both rules leave src/web/style/content.ts and both names leave their §5d bullet; bun scripts/design-lint.ts --update gives a lower or equal baseline.
• [ ] A test asserts neither class is in the page's HTML and the stage words still are; the migrate tests and bun x tsc --noEmit pass.

Before you push, run bun test test/design-lint.test.ts and bun scripts/design-lint.ts: every line they print names the file and what to change; fix each one. Striking a class from docs/DESIGN.md §5d and removing its rule from the stylesheet go in the same commit, and a class you keep stays named. Rebase onto main right before pushing: docs/DESIGN.md changes under everyone.

Files: src/web/pages/migrate.ts, src/web/client.ts, src/web/style/content.ts, docs/DESIGN.md, docs/design/baseline.json, a test. Risk: client.ts selectors; grep each name before removing it.

Parked by the crew: the check stayed red through 2 automatic tries (pull #4231 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("a hostile origin cannot reach git or the filesystem > (unnamed)") and the decider saw no point in another try (pull #4472 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #4231 closed)
2. the check failed ("a hostile origin cannot reach git or the filesystem > (unnamed)") and the decider saw no point in another try (pull #4472 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #4595. Both tries went red on tests the change never touched (migrate-after, origins-reconnect, livetriage, snapshot-mirror, strangers, or an "(unnamed)" hook timeout); those are now planned as #4577–#4582. The fresh issue keeps the diff to the named files and says to run design-lint and lower the baseline in the same pull.