webui 8: parity closure + polish + feature-complete gate

[dyoung522]

CLI parity for web-first capabilities: core batch update flow (#324) + CLI -i picker (#254); keyboard/a11y pass; visual polish; docs; the epic live review and the owner's feature-complete UI+UX sign-off gate. Part of the serve SPA redesign (#326, design doc docs/plans/2026-08-31-serve-spa-design.md). Lands on the webui feature branch.

[dyoung522]

Merged into webui as dec68e4 (--no-ff, story branch dyoung522/webui-parity, 36 commits: 8A backend/CLI parity + its reviewed fix wave, 8B SPA parity/polish/a11y/docs, and two post-gate fix waves). Closes the parity items (#324, #254, #335), renders every carried wire field, adds the keyboard-shortcuts help, the contrast/keyboard pass with ratchets, one motion language, and the consolidated docs; the Unit 5 E2E flake is closed at its root. Gate: NO-GO on three Importants → fixed → re-review found one regression → fixed and coordinator-verified. NEXT: the epic live review on webui HEAD, then the owner's feature-complete UI+UX sign-off (#326). Not released.