fix(desktop): stop refText crash on undefined composer attachment holes

A session switch or draft restore can leave undefined/null holes in the
composer attachments array. AttachmentList was guarded against this in
#49624, but the sibling submit path was not: submitPromptText maps the
same array through attachmentDisplayText/optimisticAttachmentRef and
buildContextText (a.kind / a.label / a.refText), so a hole threw
"Cannot read properties of undefined (reading 'refText')" — an uncaught
renderer error that blanks the chat pane and shows "Desktop app link
offline".

Close the whole bug class:
- attachmentDisplayText / optimisticAttachmentRef no-op on a falsy
  attachment (shared chokepoint, also protects thread.tsx drop handler).
- submitPromptText filters falsy entries from the source array, and
  buildContextText filters its (possibly post-sync) input before reading
  fields.

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

@@ -555,7 +555,14 @@ export function usePromptActions({
     async (rawText: string, options?: SubmitTextOptions) => {
       const visibleText = rawText.trim()
       const usingComposerAttachments = !options?.attachments
-      const attachments = options?.attachments ?? $composerAttachments.get()
+      // Drop undefined/null holes a session switch or draft restore can leave in
+      // the attachments array (same bug class as AttachmentList #49624). Without
+      // this, the sibling iterations below (a.kind / a.label / a.refText, and the
+      // sync step) throw "Cannot read properties of undefined (reading 'refText')"
+      // and break the chat surface.
+      const attachments = (options?.attachments ?? $composerAttachments.get()).filter(
+        (a): a is ComposerAttachment => Boolean(a)
+      )
 
       const terminalContextBlocks = terminalContextBlocksFromDraft(rawText).join('\n\n')
       const hasImage = attachments.some(a => a.kind === 'image')
@@ -568,14 +575,17 @@ export function usePromptActions({
       let attachmentRefs = attachments.map(optimisticAttachmentRef).filter((r): r is string => Boolean(r))
 
       const buildContextText = (atts: ComposerAttachment[]): string => {
-        const contextRefs = atts
+        // atts may be the post-sync array, which can reintroduce holes; filter
+        // before touching a.refText / a.kind.
+        const present = atts.filter((a): a is ComposerAttachment => Boolean(a))
+        const contextRefs = present
           .map(a => a.refText)
           .filter(Boolean)
           .join('\n')
 
         return (
           [contextRefs, terminalContextBlocks, visibleText].filter(Boolean).join('\n\n') ||
-          (atts.some(a => a.kind === 'image') ? 'What do you see in this image?' : '')
+          (present.some(a => a.kind === 'image') ? 'What do you see in this image?' : '')
         )
       }
 

apps/desktop/src/lib/chat-runtime.ts

@@ -155,6 +155,13 @@ export function pathLabel(path: string): string {
 }
 
 export function attachmentDisplayText(attachment: ComposerAttachment): string | null {
+  // Session switches / draft restores can leave undefined holes in the
+  // composer attachments array (see AttachmentList's filter(Boolean) + #49624).
+  // Every consumer funnels through here, so guard the chokepoint too.
+  if (!attachment) {
+    return null
+  }
+
   if (attachment.kind === 'terminal' && attachment.detail) {
     return `\`\`\`terminal\n${attachment.detail.trim()}\n\`\`\``
   }
@@ -188,6 +195,10 @@ export function attachmentDisplayText(attachment: ComposerAttachment): string |
  * through to `attachmentDisplayText`.
  */
 export function optimisticAttachmentRef(attachment: ComposerAttachment): string | null {
+  if (!attachment) {
+    return null
+  }
+
   if (attachment.kind === 'image' && attachment.previewUrl?.startsWith('data:')) {
     return attachment.previewUrl
   }