fix(kb): search preview shows section-in-context for text sources

User reported the search card "looks broken" when the selected hit is the
law (Wikisource source 5) — the right preview pane just duplicates the
chunk text already visible in the left list card. For PDF sources the
right pane shows the PDF page, but for text-only sources it had nothing
useful to add and fell back to a copy.

Fix:
- /kb/search hits gained chunk_index (shira-hermes b127caf), and
  /kb/source/{id}/chunks gained an `around=<chunk_index>&ctx=<k>` window.
- Espo proxy plumbing: getActionChunks reads `around`/`ctx` query
  params and forwards them; KnowledgeBaseService::sourceChunks accepts
  optional $around + $ctx and appends them to the upstream URL.
- showSearchPreview for non-PDF hits now fetches the matched chunk plus
  2 sections before and 2 after, renders them as stacked panels: the
  matched section gets a yellow #fff3cd highlight, neighbors render as
  muted context blocks. The panel header carries a prominent
  "פתח ב-Wikisource" button so users can jump to the public source page.
  After render the matched section auto-scrolls into view.
- Race protection via _previewToken: a click on a different hit while a
  fetch is in flight cancels the stale render so the user always sees
  the section that matches the currently-selected card.

PDF path unchanged.

Refs Task Master #7

manifest.json

@@ -1,7 +1,7 @@
 {
     "name": "KnowledgeBase",
     "module": "KnowledgeBase",
-    "version": "0.1.10",
+    "version": "0.1.11",
     "acceptableVersions": [
         ">=8.0.0"
     ],