feat(fristenrechner/inbox-chip): wire inbox into B2 fine-forum chips

The page-top inbox chip now drives the existing 10-bucket B2 forum
filter so search results inherit the same narrowing as the Pathway A
picker. CMS auto-selects upc_cfi + upc_coa; beA / Posteingang
auto-select de_lg + de_olg + de_bgh + de_bpatg; "Alle" clears both the
inbox and the fine chips.

State-priority discipline:

  - User chip click               always replaces activeForums with
                                  the inbox-derived set.
  - Hydrate (URL ?inbox=, no
    ?forum=)                      derives activeForums from inbox.
                                  initForumFilter (which runs first)
                                  has already loaded URL ?forum= when
                                  present, so the explicit forum= wins.
  - Hydrate (no URL ?inbox=,
    /api/me forum_pref set)       same as above: derive when ?forum=
                                  is empty.
  - popstate                      re-applies the same rule so browser
                                  back/forward stays consistent.

The "explicit URL forum= wins" rule means a colleague's CMS-narrowed
share link still works when the recipient has a different saved pref —
they see the shared narrowing, not their own.

Refs m/paliad#15.

frontend/src/client/fristenrechner.ts

@@ -2827,6 +2827,32 @@ function inboxChannelToForum(ch: InboxChannel): "upc" | "de" | null {
   return null;
 }
 
+// inboxToFineForumSlugs maps the inbox channel to the matching subset
+// of the 10-bucket B2 forum filter. CMS narrows to UPC CFI + CoA;
+// beA / Posteingang narrow to all four national-DE buckets (LG / OLG /
+// BGH / BPatG). Null returns an empty list — caller handles the
+// "no inbox set" case explicitly because the meaning differs between
+// hydrate (don't touch fine chips) and click-clear (clear them).
+function inboxToFineForumSlugs(ch: InboxChannel): string[] {
+  if (ch === "cms") return ["upc_cfi", "upc_coa"];
+  if (ch === "bea" || ch === "posteingang") return ["de_lg", "de_olg", "de_bgh", "de_bpatg"];
+  return [];
+}
+
+// applyFineForumsFromInbox replaces activeForums with the inbox's
+// implied fine-bucket set and re-renders the B2 chip strip + reissues
+// the active search. Caller decides when to invoke (user click always;
+// hydrate / popstate only when URL ?forum= is empty so an explicit
+// link-share wins over the inbox derivation).
+function applyFineForumsFromInbox(ch: InboxChannel) {
+  activeForums.clear();
+  for (const slug of inboxToFineForumSlugs(ch)) activeForums.add(slug);
+  // renderForumChips guards on missing container, so calling from
+  // contexts where Pathway B isn't yet rendered is safe.
+  renderForumChips();
+  reissueSearchWithCurrentFilters();
+}
+
 function applyInboxFilter(ch: InboxChannel) {
   const forum = inboxChannelToForum(ch);
 
@@ -2884,18 +2910,41 @@ async function initInboxFilter() {
   }
   applyInboxFilter(initial);
 
+  // Sync B2 fine-bucket chips from the inbox on hydrate ONLY when the
+  // URL doesn't explicitly carry ?forum=… — an explicit forum= comes
+  // from a shared link and should win over the user's saved inbox
+  // preference. initForumFilter (which runs first) has already
+  // populated activeForums from URL forum=, so we leave it alone here.
+  if (initial !== null && readForumsFromURL().length === 0) {
+    applyFineForumsFromInbox(initial);
+    writeForumsToURL(true);
+  }
+
   bar.querySelectorAll<HTMLButtonElement>(".fristen-inbox-chip").forEach((btn) => {
     btn.addEventListener("click", () => {
       const isClear = btn.hasAttribute("data-inbox-clear");
       const next: InboxChannel = isClear ? null : ((btn.dataset.inbox as InboxChannel) ?? null);
       writeInboxToURL(next);
       applyInboxFilter(next);
+      // User click is an explicit signal: re-narrow the B2 fine chips
+      // to match the new inbox. "Alle" clears both inbox and fine chips
+      // — that's the user's reset affordance.
+      applyFineForumsFromInbox(next);
+      writeForumsToURL(true);
       void persistInboxPref(next);
     });
   });
 
   window.addEventListener("popstate", () => {
-    applyInboxFilter(readInboxFromURL());
+    const newInbox = readInboxFromURL();
+    applyInboxFilter(newInbox);
+    // popstate can land on a URL with inbox= but no forum= (the user
+    // navigated to a state where derivation should re-apply). Don't
+    // touch activeForums when forum= is explicit — initForumFilter's
+    // own popstate handler has already loaded it from the URL.
+    if (newInbox !== null && readForumsFromURL().length === 0) {
+      applyFineForumsFromInbox(newInbox);
+    }
   });
 }