feat(t-paliad-133): Phase E — retire legacy mode tabs

m's spec lock §10 Q1 (2026-05-05): "Retire legacy tabs - we are only
resorting." This commit drops the .fristen-mode-tabs nav (Verfahrensablauf
+ Was kommt nach…) and the ?legacy=1 escape hatch. Pathway A becomes
Verfahrensablauf-only; the trigger-event panel (mode-event-panel) stays
in the DOM but is hidden by default and surfaces only via concept-card
pill drill-in (drillToTrigger flips the panels directly).

Frontend deltas:
- frontend/src/fristenrechner.tsx: drop .fristen-mode-tabs section;
  rename mode-event-panel role/label to standalone tabpanel.
- frontend/src/client/fristenrechner.ts:
  - drop isLegacyMode() + ?legacy=1 branch in showPathway().
  - drillToTrigger() now flips procedure ↔ event panels directly
    (no more #mode-event-tab click → handler chain).
  - initModeTabs() bails on tabs.length===0 (already does); no
    further changes needed.
- frontend/src/styles/global.css: drop .fristen-pathway-shell--legacy.

Backend untouched.

Build: clean. Frontend bundle 1473 keys unchanged. go build + vet +
tests pass.

The deadlines.mode.procedure / deadlines.mode.event i18n keys remain
in i18n.ts as orphans for now; cleaning them up is purely cosmetic
and lives outside the v3 scope.

frontend/src/client/fristenrechner.ts

@@ -1455,9 +1455,14 @@ function drillToProceeding(procCode: string, focusCode: string | null) {
 }
 
 function drillToTrigger(triggerId: number) {
-  const eventTab = document.getElementById("mode-event-tab");
-  if (eventTab) eventTab.click();
-  // Defer a tick so the tab activation has run before we touch event-mode state.
+  // v3 (Phase E): legacy tabs are gone. Show the event panel directly.
+  // Triggered from concept-card pill clicks; routes via Pathway A so the
+  // Verfahrensablauf user surface stays consistent.
+  const procedurePanel = document.getElementById("mode-procedure-panel");
+  const eventPanel = document.getElementById("mode-event-panel");
+  if (procedurePanel) procedurePanel.hidden = true;
+  if (eventPanel) eventPanel.hidden = false;
+  // Defer a tick so the panel swap has rendered before we touch state.
   window.setTimeout(() => {
     selectTriggerEvent(triggerId);
     document.getElementById("event-step-2")?.scrollIntoView({ behavior: "smooth", block: "start" });
@@ -1578,7 +1583,6 @@ const PATHWAY_STORAGE_KEY = "paliad.fristen.pathway";
 
 function readPathwayFromURL(): Pathway {
   const sp = new URLSearchParams(window.location.search);
-  if (sp.get("legacy") === "1") return "fork"; // legacy mode: ignore path
   const p = sp.get("path");
   if (p === "a" || p === "b") return p;
   return "fork";
@@ -1594,10 +1598,6 @@ function readBModeFromURL(): BMode {
   return "tree";
 }
 
-function isLegacyMode(): boolean {
-  return new URLSearchParams(window.location.search).get("legacy") === "1";
-}
-
 function setPathwayURL(path: Pathway, mode?: BMode, replace = false) {
   const url = new URL(window.location.href);
   if (path === "fork") {
@@ -1625,16 +1625,6 @@ function showPathway(path: Pathway, mode?: BMode) {
   const b = document.getElementById("fristen-pathway-b");
   if (!fork || !a || !b) return;
 
-  if (isLegacyMode()) {
-    // Legacy view: hide the fork + Pathway-A heading wrapper, show the
-    // wizard flat as it was pre-v3.
-    fork.hidden = true;
-    a.hidden = false;
-    a.classList.add("fristen-pathway-shell--legacy");
-    b.hidden = true;
-    return;
-  }
-
   fork.hidden = path !== "fork";
   a.hidden = path !== "a";
   b.hidden = path !== "b";

frontend/src/fristenrechner.tsx

@@ -200,12 +200,11 @@ export function renderFristenrechner(): string {
                   <span data-i18n="deadlines.pathway.a.title">Verfahrensablauf informieren</span>
                 </h2>
 
-              <div className="fristen-mode-tabs" role="tablist">
-                <button type="button" id="mode-procedure-tab" className="mode-tab is-active" role="tab" aria-selected="true" data-mode="procedure" data-i18n="deadlines.mode.procedure">Verfahrensablauf</button>
-                <button type="button" id="mode-event-tab" className="mode-tab" role="tab" aria-selected="false" data-mode="event" data-i18n="deadlines.mode.event">Was kommt nach&hellip;</button>
-              </div>
-
-              <div className="fristen-wizard mode-panel" id="mode-procedure-panel" data-mode="procedure" role="tabpanel" aria-labelledby="mode-procedure-tab">
+              {/* v3: legacy mode tabs retired (m's spec lock §10 Q1, 2026-05-05).
+                   Pathway A is Verfahrensablauf-only; trigger-event drill-in
+                   surfaces via concept-card pills with ?path=a&trigger=N URL,
+                   which resurfaces mode-event-panel programmatically below. */}
+              <div className="fristen-wizard mode-panel" id="mode-procedure-panel" data-mode="procedure" role="tabpanel">
                 <div className="wizard-step" id="step-1">
                   <h3 className="wizard-step-label">
                     <span className="step-number">1</span>
@@ -331,7 +330,7 @@ export function renderFristenrechner(): string {
                 </button>
               </div>
 
-              <div className="fristen-wizard mode-panel" id="mode-event-panel" data-mode="event" role="tabpanel" aria-labelledby="mode-event-tab" hidden>
+              <div className="fristen-wizard mode-panel" id="mode-event-panel" data-mode="event" role="tabpanel" hidden>
                 <div className="wizard-step" id="event-step-1">
                   <h3 className="wizard-step-label">
                     <span className="step-number">1</span>

frontend/src/styles/global.css

@@ -1625,17 +1625,6 @@ input[type="range"]::-moz-range-thumb {
     margin: 1.5rem 0;
 }
 
-.fristen-pathway-shell--legacy {
-    /* Pre-v3 layout for parity testing via ?legacy=1; suppresses the
-       new heading + back button so the wizard renders flat. */
-    margin: 0;
-}
-
-.fristen-pathway-shell--legacy .fristen-pathway-back,
-.fristen-pathway-shell--legacy .fristen-pathway-heading {
-    display: none;
-}
-
 .fristen-pathway-back {
     display: inline-flex;
     align-items: center;