fix(desktop): heal stale runtime-id cache + model on profile switch (#39819)

Two switch-time regressions from the multi-profile rail work:

- "Session not found" (4007): pruneSecondaryGateways idle-reaps a
  non-active profile's backend; switching back respawns a *fresh*
  backend that mints new runtime ids, but runtimeIdByStoredSessionId is
  never pruned. resumeSession's cache fast-path then makes a dead runtime
  id active and returns, so session.usage + the next prompt 404. Probe
  the cached id; on rejection drop the stale mapping and fall through to
  a full resume that rebinds a live id.

- "Forgets the LLM setting": $currentModel is a nanostore set only by
  refreshCurrentModel (gatewayState->open, etc). A swap fires
  invalidateQueries() (react-query only) and keeps the socket 'open', so
  the model/pill kept showing the previous profile. Re-pull both when
  $activeGatewayProfile changes.

apps/desktop/src/app/session/hooks/use-session-actions.ts

@@ -453,15 +453,31 @@ export function useSessionActions({
         clearComposerDraft()
         clearComposerAttachments()
 
-        void requestGateway<UsageStats>('session.usage', { session_id: cachedRuntimeId })
-          .then(usage => {
-            if (isCurrentResume() && usage) {
-              setCurrentUsage(current => ({ ...current, ...usage }))
-            }
-          })
-          .catch(() => undefined)
+        try {
+          const usage = await requestGateway<UsageStats>('session.usage', { session_id: cachedRuntimeId })
 
-        return
+          if (!isCurrentResume()) {
+            return
+          }
+
+          if (usage) {
+            setCurrentUsage(current => ({ ...current, ...usage }))
+          }
+
+          return
+        } catch {
+          // The cached runtime id was minted by a prior backend instance. A
+          // pooled profile backend that gets idle-reaped (pruneSecondaryGateways)
+          // and respawned across a profile swap mints fresh ids, so this mapping
+          // now 404s ("session not found"). Drop it and fall through to a full
+          // resume that rebinds a live runtime id.
+          if (!isCurrentResume()) {
+            return
+          }
+
+          runtimeIdByStoredSessionIdRef.current.delete(storedSessionId)
+          sessionStateByRuntimeIdRef.current.delete(cachedRuntimeId)
+        }
       }
 
       setFreshDraftReady(false)