small: The fork tree names how each fork follows its upstream in one word #11421

Each node on the fork tree page shows its follow mode (follows, proposes only, severed) as one plain word, from the same words as the fork's own page.

Done looks like

• [x] Every tree node carries the one word, read from the same source as follow_words.ts
• [x] A severed fork reads as severed, not as missing
• [x] A test fails without it; the tree page still answers under its timing test

Files expected: src/web/pages/tree_cache.ts, src/spawn/follow_words.ts, test/

Risks: do not add a query per node: read modes in the query the tree already makes.

Suggested brief: mason (a person assigns). Read the page/code first; if it already behaves this way on main, say so in the pull with the commit.

Reviewed pull #11440 at its head: ticked Every tree node carries the one word, read from the same source as follow_words.ts; A severed fork reads as severed, not as missing; A test fails without it; the tree page still answers under its timing test.