fix(csb): custom-section book picker search never worked — two latent bugs

Fine-toothed functional verification of the delegation work exposed that
the picker's book search has been dead since launch, for two
independent reasons:

- The builder fetched GET /api/books/search, a route no router has ever
  registered — every search 404'd (the CSP-dead onclick buttons on the
  results hid the failure). It now calls the unified JSON search the
  bookshelf uses (/api/media-items/search?q=&library_id=), maps its
  rows (id/title/author/cover_image_path) to the picker's BookInfo
  shape, and treats the endpoint's 404-with-results:[] as empty.

- Even with results, the picker container shared id=search-results with
  the header search dropdown (header.templ renders it on every page);
  getElementById returns the first match in tree order, so results were
  written into the invisible topbar dropdown. The picker container is
  now cs-search-results (templ + all three TS references).

Verified end-to-end at 390x844: select library -> search '1984' -> 3
results render -> click adds the selection chip -> chip x removes it.
Zero console errors.

templates/custom_section.templ

@@ -136,7 +136,7 @@ templ CustomSectionBuilder(user User, libraries []LibraryData, errorMessage stri
 							</div>
 						</div>
 						<div
-							id="search-results"
+							id="cs-search-results"
 							class="hidden mb-4 card p-3 max-h-60 overflow-y-auto space-y-2"
 							style="background-color: var(--bg-primary);"
 						></div>