feat(review-tutor): animate the configuration rail and drop diff actions on neighbour evidence

[ElbertePlinio]

Refs #62 — PR 5 of the Structure view chain, stacked on #73. Polish from Elberte's live test of #73.

What changed

• The configuration rail collapse/expand animates (grid-template-columns 200ms). Reduced motion keeps it instant via the existing global rule.
• Evidence rows whose line lives in an unchanged neighbour (CONTEXT file) no longer show Ask the tutor / Open in Diff. Those buttons had nothing to land on and failed with only a screen-reader announcement; the evidence text stays.

Tested

• vitest run packages/review-tutor/test — 263 passed; typecheck clean.
• New test: neighbour evidence has no actions, changed-file evidence keeps them; existing "outside the diff" test updated to the new contract.
• Live: Pi session on 0fca354...HEAD with neighbours on.

Not tested

• No automated visual capture of the transition; checked in a real browser only.

Risk class: standard (one Sol correctness reviewer + KISS gate).