feat(fristenrechner/inbox-chip): wire inbox into B1 cascade narrowing

Completes the #15 vision: the inbox chip now narrows the B1 decision
tree alongside Pathway A's picker and B2's fine-bucket forum filter.
Picking CMS hides DE / EPA / DPMA cascade entries; picking beA /
Posteingang hides UPC / EPA / DPMA entries. Neutral nodes (top-level
branches, Mündliche Verhandlung sub-states, court-generic events like
Ladung / Kostenfestsetzung) stay visible from every inbox setting so
the user can always reach the cross-jurisdictional middle of the tree.

Migration 065 adds paliad.event_categories.forums (text[]) with a
CHECK on {upc, de, epa, dpma}, a partial GIN index, and a two-step
backfill:

  1. Regex on slug for nodes that carry the forum token explicitly.
     Token-bounded by ^/./- so .dpma doesn't trip the de pattern.
  2. Explicit slug list for stragglers (BGH / BPatG / Versäumnisurteil /
     Hinweisbeschluss are DE-only; r116-eingaben is EPA-only).

NULL stays neutral. Migration applied to live Supabase; tracker at v65.

Backend: EventCategoryNode JSON gains an optional `forums` array;
EventCategoryService.Tree SELECT includes the column and threads it
through to the response.

Frontend: new module-level currentInboxChannel mirrors the chip state
so renderB1Cascade can ask "which forum is active?" without re-deriving
from the URL on every step. inboxFilterAllowsForums(forums) gates each
child node — neutral arrays (undefined / empty) always pass; tagged
arrays must include the active forum. applyInboxFilter re-renders the
cascade so chip clicks reflow B1 in place. Pathway A picker filter
and B2 fine-bucket sync remain orthogonal — same chip, three filters.

Refs m/paliad#15 (B1 follow-up).

frontend/src/client/fristenrechner.ts

@@ -2409,6 +2409,9 @@ interface EventCategoryNode {
   icon?: string;
   sort_order: number;
   is_leaf: boolean;
+  // #15 follow-up: coarse forum tags ('upc' | 'de' | 'epa' | 'dpma').
+  // Empty / undefined = neutral, node visible from every inbox setting.
+  forums?: string[];
   children?: EventCategoryNode[];
 }
 
@@ -2494,7 +2497,12 @@ function renderB1Cascade(currentSlug: string) {
 
   const trail = buildBreadcrumb(eventCategoryTree, currentSlug);
   const node = trail.length > 0 ? trail[trail.length - 1] : null;
-  const childScope = node ? (node.children || []) : eventCategoryTree;
+  const rawChildScope = node ? (node.children || []) : eventCategoryTree;
+  // #15 follow-up: drop children whose forums tag doesn't match the
+  // active inbox channel. Nodes with no forums (neutral) stay visible
+  // so the user always reaches court-event / Schriftsatz parents that
+  // span jurisdictions.
+  const childScope = rawChildScope.filter((c) => inboxFilterAllowsForums(c.forums));
 
   const breadcrumbHtml = trail.length === 0
     ? ""
@@ -2804,6 +2812,13 @@ type InboxChannel = "cms" | "bea" | "posteingang" | null;
 
 const INBOX_CHANNEL_VALUES = new Set<string>(["cms", "bea", "posteingang"]);
 
+// currentInboxChannel mirrors the chip's active state so the B1 cascade
+// renderer (which lives in a different section of this file) can ask
+// "which forum is active right now?" without re-deriving from URL on
+// every render. Updated by applyInboxFilter on hydrate / click /
+// popstate.
+let currentInboxChannel: InboxChannel = null;
+
 function readInboxFromURL(): InboxChannel {
   const raw = new URLSearchParams(window.location.search).get("inbox");
   return raw && INBOX_CHANNEL_VALUES.has(raw) ? (raw as InboxChannel) : null;
@@ -2854,6 +2869,7 @@ function applyFineForumsFromInbox(ch: InboxChannel) {
 }
 
 function applyInboxFilter(ch: InboxChannel) {
+  currentInboxChannel = ch;
   const forum = inboxChannelToForum(ch);
 
   document.querySelectorAll<HTMLButtonElement>(".fristen-inbox-chip").forEach((btn) => {
@@ -2871,6 +2887,25 @@ function applyInboxFilter(ch: InboxChannel) {
     const groupForum = g.dataset.forum || "";
     g.hidden = forum !== null && groupForum !== forum;
   });
+
+  // Re-render the B1 cascade so its button set picks up the new forum
+  // narrowing. Render is a no-op if the tree hasn't loaded yet or the
+  // cascade DOM isn't mounted (Pathway B not visible) — both guards
+  // already inside renderB1Cascade.
+  if (eventCategoryTree) {
+    renderB1Cascade(readB1PathFromURL());
+  }
+}
+
+// inboxFilterAllowsForums returns true when a node with the given
+// forums tags should be visible under the current inbox chip. Neutral
+// nodes (forums undefined / empty) are always visible. When no inbox
+// is active, every node is visible.
+function inboxFilterAllowsForums(forums: string[] | undefined): boolean {
+  if (!forums || forums.length === 0) return true;
+  const active = inboxChannelToForum(currentInboxChannel);
+  if (active === null) return true;
+  return forums.includes(active);
 }
 
 async function persistInboxPref(ch: InboxChannel) {