fix(ui): wire up collection detail page interactions

The /collections/:id page had several broken features because three
referenced functions (removeBook, toggleBookForRemoval,
filterCollectionBooks) were never defined, and every book card was
wrapped in <a href="/media/..."> so clicking the checkbox or remove
button navigated to the book detail page instead.

Card restructure:
- Remove the <a> wrapper; title and cover are now individual links.
- Checkbox sits in a <label> with expanded click area (p-2 -m-2).
- Checkbox uses Alpine :checked/@change bound to a reactive
  selectedBooks array on the collections component.

Remove (single + bulk):
- Add removeBook(id) and bulkRemove() methods with confirm() dialogs.
- Wire the "Remove Selected" button with :disabled binding and @click.
- Selected-count badge is now Alpine-reactive (x-show/x-text).

Search within collection:
- Add filterCollectionBooks() that filters cards client-side by
  title/author via data-* attributes and @input.

Book picker ("Add Books"):
- Point the HTMX search inputs at the existing /api/media-items/search
  endpoint instead of the non-existent /api/media-items/filtered.
- Add hx-trigger="loadBooks" + hx-get to the grid so loadBooks()
  actually fires an initial request when the picker opens.
- Merge the hidden limit/offset inputs into the #book-picker-filters
  div so hx-include picks them up (was a separate <form id=filter-form>
  that nobody referenced).
- Add show_checkbox mode to handleSearchHTML: when present, render a
  new BookPickerGrid template with clickable, selectable cards instead
  of the reader BookCard.
- Fix bookPicker submit() to location.reload() instead of a non-existent
  reloadCollection HTMX event, and clearFilters() to target text inputs.

internal/router/search.go

@@ -114,9 +114,13 @@ func handleSearchHTML(c *echo.Context, cfg *Config) error {
 	}
 	// Stamp active conflict flags so cards route the play action correctly
 	bookInfoList = handlers.MarkActiveConflicts(c.Request().Context(), cfg.Queries, user.ID, bookInfoList)
-	// Render using BooksGrid template
+	// Render using BooksGrid template (or BookPickerGrid for collection picker)
 	var buf bytes.Buffer
-	err = templates.BooksGrid(bookInfoList, limit, offset, totalCount, libraryID).Render(c.Request().Context(), &buf)
+	if c.QueryParam("show_checkbox") == "true" {
+		err = templates.BookPickerGrid(bookInfoList).Render(c.Request().Context(), &buf)
+	} else {
+		err = templates.BooksGrid(bookInfoList, limit, offset, totalCount, libraryID).Render(c.Request().Context(), &buf)
+	}
 	if err != nil {
 		log.Printf("Template render error: %v", err)
 		return c.HTML(http.StatusInternalServerError, `<div style="color: red;">Render error</div>`)