Workspace search dialog: livesearch and AI answers (#426)

Turn the workspace search popup into the approved dialog design
(internal ticket #426):

- Live results while typing (kitconcept.solr @solr-suggest, debounced),
  with type icons, dates and click-to-navigate.
- "Ask AI" (KI fragen) runs the existing @Rag-Search pipeline and
  renders the AI Overview (KI-Überblick) panel: Beta badge, grounded
  answer with typewriter reveal and markdown emphasis, source chips,
  inactive follow-up field (multi-turn backend pending) and the
  AI-generated disclaimer. AI errors surface as a friendly localized
  message; the technical detail goes to the console only (raw messages
  like embed timeouts under concurrent LLM load, internal ticket #515).
  Only shown when the backend reports kitconcept.solr.rag_available.
- The filter chips are working-looking dropdowns with the option lists
  from the approved design; selections are remembered visually but do
  not filter - the backing facets (and the workspace scope switch) are
  deferred (accepted demo scope).
- Search page (override): the AI answer area and the @Rag-Search
  request are removed entirely - the AI lives in the workspace dialog
  only, so pages outside it never occupy a backend worker with an LLM
  call (internal ticket #515). Also fixes the Segment children
  prop-types warning for the Toolbar portal (kitconcept.solr#97).
- Cmd+K / Ctrl+K opens the dialog; state resets on close. German
  translations for all new UI strings.

Author: reekitconcept