feat: show draft preset chips under the welcome message on mobile/vscode

On narrow surfaces (mobile, vscode) the composer sits at the bottom and
only the welcome message is centered, so the preset chips had nowhere
sensible to live and were desktop-only. Render them under the centered
welcome message there instead.

Extract the shared preset list (draftPresets.ts) and chip row
(DraftPresetChips) so both layouts reuse them. Since the command-aware
submit lives in ChatInput, ChatContainer triggers it through a new
input-store channel (requestPresetSubmit / consumePendingPresetSubmit)
that ChatInput consumes. Mini-chat stays excluded — too small for the row.

packages/ui/src/sync/input-store.ts

@@ -88,11 +88,19 @@ export type InputState = {
   pendingInputText: string | null
   pendingInputMode: "replace" | "append" | "append-inline"
   pendingSyntheticParts: SyntheticContextPart[] | null
+  /**
+   * Text a draft preset chip asked to submit immediately. Set by surfaces that
+   * render the chips outside ChatInput (e.g. under the welcome message on
+   * narrow layouts); consumed by ChatInput, which owns the command-aware submit.
+   */
+  pendingPresetSubmit: string | null
   attachedFiles: AttachedFile[]
   activeEditorFile: VSCodeActiveEditorFile | null
 
   setPendingInputText: (text: string | null, mode?: "replace" | "append" | "append-inline") => void
   consumePendingInputText: () => { text: string; mode: "replace" | "append" | "append-inline" } | null
+  requestPresetSubmit: (text: string) => void
+  consumePendingPresetSubmit: () => string | null
   setPendingSyntheticParts: (parts: SyntheticContextPart[] | null) => void
   consumePendingSyntheticParts: () => SyntheticContextPart[] | null
   addAttachedFile: (file: File) => Promise<void>
@@ -110,6 +118,7 @@ export const useInputStore = create<InputState>()((set, get) => ({
   pendingInputText: null,
   pendingInputMode: "replace",
   pendingSyntheticParts: null,
+  pendingPresetSubmit: null,
   attachedFiles: [],
   activeEditorFile: null,
 
@@ -123,6 +132,15 @@ export const useInputStore = create<InputState>()((set, get) => ({
     return { text: pendingInputText, mode: pendingInputMode }
   },
 
+  requestPresetSubmit: (text) => set({ pendingPresetSubmit: text }),
+
+  consumePendingPresetSubmit: () => {
+    const { pendingPresetSubmit } = get()
+    if (pendingPresetSubmit === null) return null
+    set({ pendingPresetSubmit: null })
+    return pendingPresetSubmit
+  },
+
   setPendingSyntheticParts: (parts) => set({ pendingSyntheticParts: parts }),
 
   consumePendingSyntheticParts: () => {