Citation badges: hover/focus preview + a11y (#1466) - #2045

Summary

Part of #1466. Extends the existing citation UI (<cite type="s|r">N</cite> pills rendered by
client/src/utils/citationTransformer.js) with a real hover/focus preview and better accessibility,
matching two of the issue's acceptance criteria:

• Hover preview: badges now carry a short excerpt of the passage they cite (from
  citations.references[].content) or the document's title (from citations.resultItems[].title
  for whole-document citations), shown in a small custom tooltip on mouse hover and keyboard
  focus (native title tooltips don't fire on focus, so this is also an a11y improvement, not
  just cosmetic).
• Accessible name: each badge's aria-label now includes that excerpt (e.g. "Source 1: The sky is blue because of...") instead of a bare "Source 1", so screen reader users hear what a
  citation actually refers to.
• Source panel highlight on hover: hovering/focusing a badge briefly highlights the
  corresponding entry in CitationPanel.jsx (same ring style already used for click-to-scroll) —
  but without auto-scrolling or auto-expanding a collapsed passage, so it can't yank the viewport
  around. Clicking a badge keeps its existing scroll-and-expand behavior unchanged.
• Keyboard focus ring: .citation-badge:focus-visible now has a visible outline (previously
  only present in an unused, never-imported CSS string export — the real, wired-up
  StreamingMarkdown.css had no focus-visible affordance at all).

Excerpt text is HTML-attribute-escaped and truncated to 160 characters before being embedded.

Scope note

This issue also proposes an opt-in answer-verification pass (a second LLM call flagging
claims not supported by the cited sources) and a citation-format change. Per the implementation
plan already posted on the issue, that half has several open, genuinely blocking design questions
(model selection/cost accounting, persistence across reloads, whether it should also cover
web-search-derived claims) that need a product decision before implementation. This PR intentionally
covers only the citation-UI polish, which needed no such decision — the verification pass is left
as a follow-up.

Test plan

• New client/src/utils/citationTransformer.test.js (plain-node test, matches this repo's
  existing client-side test convention) covering: excerpt lookup for both citation types,
  truncation at 160 chars, HTML-attribute escaping, and the no-citations-data fallback. All 9
  assertions pass (node client/src/utils/citationTransformer.test.js).
• npx eslint on all changed files — 0 errors (pre-existing warnings only, unrelated to this
  change).
• npx prettier --write on all changed files.
• npx vite build succeeds.
• Manual browser verification of the hover/focus tooltip and panel highlight was not performed
  in this sandboxed session.

Changelog entry added under docs/releases/5.5.0/features.md per this repo's convention.

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