feat(fristenrechner): Slice S5 — flip overhaul default; legacy under ?legacy=1 (m/paliad#146)

`isOverhaulMode()` now returns true unless the URL carries
`?legacy=1`. The overhaul UI from S2-S4 (mode tabs + Mode A
search + Mode B wizard + shared result view) becomes the default
landing for /tools/fristenrechner; the legacy three-step wizard +
Pathway A/B + cascade is reachable only via the explicit
`?legacy=1` opt-out for the two-week deprecation window before
S6 drops the legacy code paths entirely.

The pre-existing `?overhaul=1` deep links from S2-S4 still
resolve — the detector treats *absence* of `?legacy=1` as
overhaul, so bookmarks stay valid. No sidebar / header / outbound
link change needed: those all point at the bare
`/tools/fristenrechner` URL, which now boots overhaul.

Verified — bun build clean (2971 i18n keys unchanged), 256
frontend tests pass, go build + vet clean.

frontend/src/client/fristenrechner-result.ts

@@ -80,11 +80,14 @@ let currentProjectId: string | null = null;
 
 // Public API ----------------------------------------------------------
 
-// isOverhaulMode reports whether the page is in overhaul mode (S2+).
-// True when `?overhaul=1` is present. Once S5 flips the flag, the
-// reverse check (?legacy=1) replaces this.
+// isOverhaulMode reports whether the page is in overhaul mode.
+// After Slice S5 (t-paliad-323), overhaul is the default; the legacy
+// wizard / row-stack / cascade is only reachable via `?legacy=1` for
+// a two-week deprecation window. The `?overhaul=1` deep links from
+// S2-S4 still work — they're now redundant with the default but kept
+// alive so bookmarks don't 302 / lose state.
 export function isOverhaulMode(): boolean {
-  return new URLSearchParams(window.location.search).get("overhaul") === "1";
+  return new URLSearchParams(window.location.search).get("legacy") !== "1";
 }
 
 // resolveProjectId reads the active Akte from the URL query string.

frontend/src/client/fristenrechner.ts

@@ -701,15 +701,16 @@ document.addEventListener("DOMContentLoaded", () => {
   initI18n();
   initSidebar();
 
-  // t-paliad-323 Slice S2 — Fristenrechner overhaul boot.
-  // When ?overhaul=1 is set, hide the legacy three-step wizard /
-  // Pathway A+B shells and mount the new result view in their place.
-  // Deep-linkable via ?overhaul=1&event=<code>&trigger_date=…&project=…
-  // (the trigger date defaults to today when omitted). S3 (Mode A
-  // search) and S4 (Mode B wizard) will land users here once they
-  // identify a trigger event — for now the surface is reached only
-  // via deep link, but ?overhaul=1 alone shows the empty shell so
-  // the path is exercisable end-to-end.
+  // t-paliad-323 Slice S5 — Fristenrechner overhaul is the default
+  // boot path. The legacy three-step wizard / Pathway A+B shells are
+  // reachable only via `?legacy=1` for a two-week deprecation window
+  // after this commit lands. Deep-linkable via
+  //   /tools/fristenrechner?event=<code>&trigger_date=…&project=…
+  // for the result view, or via
+  //   /tools/fristenrechner                  → Mode A (search) tab
+  //   /tools/fristenrechner?mode=wizard     → Mode B wizard tab
+  // The `?overhaul=1` deep links from S2-S4 still resolve (the
+  // detector treats absence of `?legacy=1` as "overhaul").
   if (isOverhaulMode()) {
     bootOverhaulMode();
     return;