refactor(desktop): make composer model picker sticky session state

The picker no longer touches the profile default. Model/effort/fast live as
plain UI state persisted in localStorage, so a pick follows across Cmd+N and
restarts instead of snapping back. New chats ship that state through
session.create as per-session overrides; live chats still scope switches to the
current session. Settings -> Model remains the only surface that writes the
profile default.

The gateway now accepts those session.create overrides, builds the agent with
them directly, reflects them in the immediate session.info payload, and writes
the chat's own model_config into the lazy DB row so reconnect/resume restores
that chat instead of the global default.

apps/desktop/src/components/model-picker.tsx

@@ -11,7 +11,6 @@ import { startManualOnboarding } from '../store/onboarding'
 
 import { InlineNotice } from './notifications'
 import { Button } from './ui/button'
-import { Checkbox } from './ui/checkbox'
 import { Command, CommandEmpty, CommandGroup, CommandInput, CommandItem, CommandList } from './ui/command'
 import { Dialog, DialogContent, DialogDescription, DialogFooter, DialogHeader, DialogTitle } from './ui/dialog'
 import { Skeleton } from './ui/skeleton'
@@ -23,7 +22,7 @@ interface ModelPickerDialogProps {
   sessionId?: string | null
   currentModel: string
   currentProvider: string
-  onSelect: (selection: { provider: string; model: string; persistGlobal: boolean }) => void
+  onSelect: (selection: { provider: string; model: string }) => void
   /**
    * Optional class to apply to DialogContent. Use to override z-index when
    * stacking the picker on top of another fixed overlay (e.g. the desktop
@@ -45,7 +44,6 @@ export function ModelPickerDialog({
 }: ModelPickerDialogProps) {
   const { t } = useI18n()
   const copy = t.modelPicker
-  const [persistGlobal, setPersistGlobal] = useState(!sessionId)
   // Own the search term so we can filter manually. cmdk's built-in
   // shouldFilter reorders items by its fuzzy-match score (≈alphabetical with
   // an empty query), which destroys the backend's curated order. We disable
@@ -79,11 +77,7 @@ export function ModelPickerDialog({
     : null
 
   const selectModel = (provider: ModelOptionProvider, model: string) => {
-    onSelect({
-      provider: provider.slug,
-      model,
-      persistGlobal: persistGlobal || !sessionId
-    })
+    onSelect({ provider: provider.slug, model })
     onOpenChange(false)
   }
 
@@ -128,24 +122,13 @@ export function ModelPickerDialog({
           </CommandList>
         </Command>
 
-        <DialogFooter className="flex-row items-center justify-between gap-3 bg-card p-3 sm:justify-between">
-          <label className="flex cursor-pointer select-none items-center gap-2 text-xs text-muted-foreground">
-            <Checkbox
-              checked={persistGlobal || !sessionId}
-              disabled={!sessionId}
-              onCheckedChange={checked => setPersistGlobal(checked === true)}
-            />
-            {sessionId ? copy.persistGlobalSession : copy.persistGlobal}
-          </label>
-
-          <div className="flex items-center gap-2">
-            <Button onClick={addProvider} variant="ghost">
-              {copy.addProvider}
-            </Button>
-            <Button onClick={() => onOpenChange(false)} variant="outline">
-              {t.common.cancel}
-            </Button>
-          </div>
+        <DialogFooter className="flex-row items-center justify-end gap-2 bg-card p-3">
+          <Button onClick={addProvider} variant="ghost">
+            {copy.addProvider}
+          </Button>
+          <Button onClick={() => onOpenChange(false)} variant="outline">
+            {t.common.cancel}
+          </Button>
         </DialogFooter>
       </DialogContent>
     </Dialog>