feat(t-paliad-134): B1 surface — render concept cards beneath decision tree

Pathway B B1 mode previously rendered an empty result area on every
state — the runB1Search() output target was #fristen-search-results,
which lives inside the B2 panel. When B2 is hidden (B1 active), the
results were written into a hidden subtree and never seen.

Changes:
- TSX: add #fristen-b1-results inside #fristen-b1-panel, below the
  cascade button row.
- frontend/fristenrechner.ts: extract renderSearchResultsInto() and
  wirePillClicks(); runB1Search now writes to fristen-b1-results,
  fetches /api/.../search?browse=all when no slug is picked yet (full
  landscape on entry), and applies CSS-driven loading dim with a seq
  guard against out-of-order responses. Hoisted loadAndRenderB1() so
  showBMode("tree") can trigger the tree load on Pathway B entry
  (radio.checked = true does not fire change events).
- backend: SearchOptions.BrowseAll, allMappedConceptIDs() returning
  the union of every concept reachable from any leaf via
  paliad.event_category_concepts, lifted limit ceiling for browse
  modes (default 200, max 500). Handler exposes ?browse=all.
- CSS: shared loading-state styling for fristen-b1-results.

internal/handlers/fristenrechner_search.go

@@ -22,11 +22,16 @@ import (
 //                         from this taxonomy node and its descendants
 //                         appear. Empty q is allowed when this is set
 //                         (browse mode).
+//   browse              - "all" enables v3 B1 entry mode: returns
+//                         every concept mapped to any leaf of the
+//                         decision tree (no narrowing, no query).
+//                         Ignored when q is non-empty.
 //   forum               - comma-separated v3 forum-bucket slugs
 //                         (upc_cfi, upc_coa, de_lg, de_olg, de_bgh,
 //                         de_bpatg, epa_grant, epa_opp, epa_appeal,
 //                         dpma). Trigger pills bypass this filter.
-//   limit               - max cards (default 12, max 30)
+//   limit               - max cards (default 12, max 30; in browse
+//                         modes default 200, max 500)
 //
 // Returns an empty cards array (not 400) when q is empty — that lets
 // the frontend boot the search input without a server round-trip.
@@ -44,6 +49,7 @@ func handleFristenrechnerSearch(w http.ResponseWriter, r *http.Request) {
 		Source:            r.URL.Query().Get("source"),
 		EventCategorySlug: r.URL.Query().Get("event_category_slug"),
 		Forums:            parseCSV(r.URL.Query().Get("forum")),
+		BrowseAll:         r.URL.Query().Get("browse") == "all",
 		Limit:             parseLimit(r.URL.Query().Get("limit")),
 	}
 	resp, err := dbSvc.deadlineSearch.Search(r.Context(), q, opts)

internal/services/deadline_search_service.go

@@ -83,6 +83,12 @@ type SearchOptions struct {
 	// v3 (t-paliad-133):
 	EventCategorySlug string   // drives B1 decision-tree narrowing
 	Forums            []string // multi-select forum buckets (UNION within)
+	// v3 (t-paliad-134): explicit "browse everything" mode for B1 entry,
+	// before the user has picked any tree node. Returns every concept
+	// that is mapped to any leaf via paliad.event_category_concepts —
+	// i.e. the full landscape of B1-reachable concepts. q must be empty
+	// when BrowseAll is true; ignored otherwise.
+	BrowseAll bool
 	Limit    int
 	MaxLimit int
 }
@@ -201,12 +207,24 @@ type pillRow struct {
 // decision-tree cascade in Pathway B).
 func (s *DeadlineSearchService) Search(ctx context.Context, q string, opts SearchOptions) (*SearchResponse, error) {
 	limit := opts.Limit
-	if limit <= 0 {
-		limit = 12
-	}
 	maxLimit := opts.MaxLimit
-	if maxLimit <= 0 {
-		maxLimit = 30
+	// Browse mode (B1: slug-driven or all-mapped) returns the entire
+	// reachable concept set, which exceeds the trigram-search defaults.
+	// Lift the ceiling so the user sees the full landscape on entry.
+	if opts.BrowseAll || opts.EventCategorySlug != "" {
+		if limit <= 0 {
+			limit = 200
+		}
+		if maxLimit <= 0 {
+			maxLimit = 500
+		}
+	} else {
+		if limit <= 0 {
+			limit = 12
+		}
+		if maxLimit <= 0 {
+			maxLimit = 30
+		}
 	}
 	if limit > maxLimit {
 		limit = maxLimit
@@ -219,9 +237,12 @@ func (s *DeadlineSearchService) Search(ctx context.Context, q string, opts Searc
 	}
 
 	qNorm := normalizeQuery(q)
-	browseMode := qNorm == "" && opts.EventCategorySlug != ""
+	browseMode := qNorm == "" && (opts.EventCategorySlug != "" || opts.BrowseAll)
 
 	// v3: resolve the event-category slug to a concept_id allow-list.
+	// When BrowseAll is set without a slug, the allow-list is the union
+	// of every concept reachable from any leaf — i.e. all rows of
+	// paliad.event_category_concepts.
 	var allowConceptIDs []string
 	if opts.EventCategorySlug != "" && s.eventCategory != nil {
 		ids, err := s.eventCategory.ConceptIDsForSlug(ctx, opts.EventCategorySlug)
@@ -234,6 +255,15 @@ func (s *DeadlineSearchService) Search(ctx context.Context, q string, opts Searc
 			return resp, nil
 		}
 		allowConceptIDs = ids
+	} else if opts.BrowseAll {
+		ids, err := s.allMappedConceptIDs(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if len(ids) == 0 {
+			return resp, nil
+		}
+		allowConceptIDs = ids
 	}
 
 	// v3: translate forum slugs to proceeding_code allow-list.
@@ -279,6 +309,19 @@ func (s *DeadlineSearchService) Search(ctx context.Context, q string, opts Searc
 	return resp, nil
 }
 
+// allMappedConceptIDs returns every distinct concept_id that has at
+// least one row in paliad.event_category_concepts — the universe of
+// concepts reachable from any leaf of the v3 decision tree. Drives B1
+// browse-all mode (no slug picked yet, show the full landscape).
+func (s *DeadlineSearchService) allMappedConceptIDs(ctx context.Context) ([]string, error) {
+	const sqlText = `SELECT DISTINCT concept_id::text FROM paliad.event_category_concepts`
+	var ids []string
+	if err := s.db.SelectContext(ctx, &ids, sqlText); err != nil {
+		return nil, fmt.Errorf("all mapped concept ids: %w", err)
+	}
+	return ids, nil
+}
+
 // translateForums maps a list of forum slugs to the union of their
 // proceeding_type_codes via ForumToProceedingCodes. Unknown slugs are
 // silently dropped.