fix(desktop): forget in-memory paging state on a connection/mode re-home

wipeSessionListsForGatewaySwitch cleared the persisted transcript-tail
cache but never the in-memory $transcriptTailBySessionId atom. Before this
stack both an ambient local read and an ambient remote read of a recycled
stored id landed under the bare-id key, so the newer backend simply
overwrote the older. Owner-keyed entries coexist instead, and the
unique-match lookup that gates "Show earlier" sees two candidates and
fails closed for that session until eviction. Wipe the atom alongside the
cache.

apps/desktop/src/store/gateway-switch.test.ts

@@ -18,6 +18,7 @@ import {
   setSessionsLoading
 } from '@/store/session'
 import { $stalledSessionIds } from '@/store/session-states'
+import { $transcriptTailBySessionId, recordTranscriptTail, transcriptTailState } from '@/store/transcript-tail'
 
 import {
   $gatewaySwitching,
@@ -79,6 +80,28 @@ describe('wipeSessionListsForGatewaySwitch', () => {
     expect($freshDraftReady.get()).toBe(true)
   })
 
+  it("forgets the previous backend's in-memory paging state", () => {
+    const page = {
+      messages: Array.from({ length: 120 }, (_, index) => ({
+        id: index,
+        role: 'user' as const,
+        content: '',
+        timestamp: 1
+      })),
+      pagination: { limit: 120, offset: 0, order: 'latest' as const, returned: 120 }
+    }
+
+    recordTranscriptTail('recycled-id', page, { connectionId: 'local', profile: 'default' })
+
+    wipeSessionListsForGatewaySwitch()
+
+    // A same-id session on the next backend must resolve its own tail alone.
+    recordTranscriptTail('recycled-id', page, { connectionId: 'remote-1', profile: 'default' })
+    expect(Object.keys($transcriptTailBySessionId.get())).toHaveLength(1)
+    expect(transcriptTailState('recycled-id')?.possiblyTruncated).toBe(true)
+    $transcriptTailBySessionId.set({})
+  })
+
   it('strands in-flight profile-list fetches so the old backend cannot repaint the rail (#85731)', () => {
     // The soft re-home moves /api/profiles routing to the NEW backend; a
     // response still in flight from the previous one must be invalidated

apps/desktop/src/store/gateway-switch.ts

@@ -26,6 +26,7 @@ import {
 import { clearAllSessionControl } from '@/store/session-control'
 import { resetSessionPinMirror } from '@/store/session-pin-sync'
 import { clearAllSessionStates } from '@/store/session-states'
+import { clearAllTranscriptTails } from '@/store/transcript-tail'
 import { clearTranscriptTails } from '@/store/transcript-tail-cache'
 
 // True while a connection switch is mid-flight — a Settings → Gateway apply
@@ -227,8 +228,12 @@ export function wipeSessionListsForGatewaySwitch(): void {
 
   // Cached transcript tails belong to the PREVIOUS backend's sessions; a
   // different backend can recycle stored ids, and painting another machine's
-  // conversation under a same-named id is worse than a loader. Wipe them.
+  // conversation under a same-named id is worse than a loader. Wipe both the
+  // persisted cache and the in-memory paging entries — the latter are keyed by
+  // owner, so a survivor from the old backend would sit beside the new one and
+  // fail the unique-match lookup that shows "Show earlier".
   clearTranscriptTails()
+  clearAllTranscriptTails()
 
   // Narrowed: account/marketplace/onboarding caches are global, not gateway-
   // scoped, so a mode swap must not refetch them.

apps/desktop/src/store/transcript-tail.ts

@@ -178,6 +178,13 @@ export function transcriptTailState(
   return matches.length === 1 ? matches[0][1] : undefined
 }
 
+/** Forget every tail: the backend behind the window changed and a recycled
+ *  stored id must not carry the previous backend's paging state. */
+export function clearAllTranscriptTails(): void {
+  transcriptTailOrder = []
+  $transcriptTailBySessionId.set({})
+}
+
 export function clearTranscriptTail(storedSessionId: string, profile?: TranscriptProfileScope): void {
   const current = $transcriptTailBySessionId.get()