fix: stream conversation pages progressively to unblock Export All UI

With 400+ conversations the old code made 4+ sequential requests and
only rendered anything after all of them completed, freezing the dialog
for 10+ seconds with no interactable elements.

- Add optional `onBatch` callback to `fetchAllConversations` that fires
  after each page arrives, so callers can react incrementally
- Wire `onBatch` in ExportDialog to append each batch to state as it
  lands — first 100 conversations appear almost instantly
- Remove `loading` from the `disabled` guard so the user can select and
  export already-loaded conversations while remaining pages still fetch
- Show "Loading…" only when the list is empty (initial blank state);
  replace with a subtle italic sentinel at the bottom of the list while
  subsequent pages are still in flight
- Allow the project selector to be changed at any time during loading

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api.ts

@@ -548,7 +548,7 @@ async function fetchProjectConversations(project: string, cursor: string | numbe
     }
 }
 
-export async function fetchAllConversations(project: string | null = null, maxConversations = 1000): Promise<ApiConversationItem[]> {
+export async function fetchAllConversations(project: string | null = null, maxConversations = 1000, onBatch?: (batch: ApiConversationItem[]) => void): Promise<ApiConversationItem[]> {
     const conversations: ApiConversationItem[] = []
     const limit = project === null ? 100 : 50 // gizmos api uses a smaller limit
     let offset = 0
@@ -565,6 +565,7 @@ export async function fetchAllConversations(project: string | null = null, maxCo
             }
             conversations.push(...result.items)
             if (result.items.length === 0) break
+            onBatch?.(result.items)
             // Stop if the API signals no more pages (no total count and no next cursor)
             if (result.total == null && result.cursor == null) break
             // Stop if we've reached the total reported by the API OR the user-defined limit

src/ui/ExportDialog.tsx

@@ -77,10 +77,9 @@ const ConversationSelect: FC<ConversationSelectProps> = ({
                 />
             </div>
             <ul className="SelectList">
-                {loading && <li className="SelectItem">{t('Loading')}...</li>}
+                {loading && conversations.length === 0 && <li className="SelectItem">{t('Loading')}...</li>}
                 {error && <li className="SelectItem">{t('Error')}: {error}</li>}
-                {!loading && !error
-                && conversations.map(c => (
+                {conversations.map(c => (
                     <li className="SelectItem" key={c.id}>
                         <CheckBox
                             label={c.title}
@@ -95,6 +94,9 @@ const ConversationSelect: FC<ConversationSelectProps> = ({
                         />
                     </li>
                 ))}
+                {loading && conversations.length > 0 && (
+                    <li className="SelectItem" style={{ opacity: 0.5, fontStyle: 'italic' }}>{t('Loading')}...</li>
+                )}
             </ul>
         </>
     )
@@ -131,7 +133,7 @@ const DialogContent: FC<DialogContentProps> = ({ format }) => {
 
     const [selected, setSelected] = useState<ApiConversationItem[]>([])
     const [exportType, setExportType] = useState(exportAllOptions[0].label)
-    const disabled = loading || processing || !!error || selected.length === 0
+    const disabled = processing || !!error || selected.length === 0
 
     const requestQueue = useMemo(() => new RequestQueue<ApiConversationWithId>(200, 1600), [])
     const archiveQueue = useMemo(() => new RequestQueue<boolean>(200, 1600), [])
@@ -295,9 +297,13 @@ const DialogContent: FC<DialogContentProps> = ({ format }) => {
     }, [])
 
     useEffect(() => {
+        setApiConversations([])
         setLoading(true)
-        fetchAllConversations(selectedProject?.id, exportAllLimit)
-            .then(setApiConversations)
+        fetchAllConversations(
+            selectedProject?.id,
+            exportAllLimit,
+            batch => setApiConversations(prev => [...prev, ...batch]),
+        )
             .catch((err) => {
                 console.error('Error fetching conversations:', err)
                 setError(err.message || 'Failed to load conversations')
@@ -328,7 +334,7 @@ const DialogContent: FC<DialogContentProps> = ({ format }) => {
                     {t('Export from API')}
                 </div>
             )}
-            <ProjectSelect projects={projects} selected={selectedProject} setSelected={setSelectedProject} disabled={processing || loading} />
+            <ProjectSelect projects={projects} selected={selectedProject} setSelected={setSelectedProject} disabled={processing} />
             <ConversationSelect
                 conversations={conversations}
                 selected={selected}