refactor(desktop): extract composer placeholder logic into useComposerPlaceholder

Moves the resting-placeholder state + the conversation-change re-roll effect +
the disabled/reconnecting/starting derivation out of ChatBar into
hooks/use-composer-placeholder.ts, verbatim. The hook owns its own i18n + browse
reset; ChatBar just reads the derived string.

apps/desktop/src/app/chat/composer/hooks/use-composer-placeholder.ts

@@ -0,0 +1,60 @@
+import { useEffect, useRef, useState } from 'react'
+
+import { useI18n } from '@/i18n'
+import { resetBrowseState } from '@/store/composer-input-history'
+
+import { pickPlaceholder } from '../composer-utils'
+
+interface UseComposerPlaceholderOptions {
+  disabled: boolean
+  reconnecting: boolean
+  sessionId: null | string | undefined
+}
+
+/**
+ * The composer's placeholder text. A resting starter (new session) / continuation
+ * (existing session) is picked once and only re-rolled when we genuinely move to
+ * a *different* conversation — the null→id persist of a freshly-started session
+ * keeps its starter so the text doesn't flip mid-stream. While the transport is
+ * down, it swaps to a reconnecting / starting message instead.
+ */
+export function useComposerPlaceholder({ disabled, reconnecting, sessionId }: UseComposerPlaceholderOptions): string {
+  const { t } = useI18n()
+  const newSessionPlaceholders = t.composer.newSessionPlaceholders
+  const followUpPlaceholders = t.composer.followUpPlaceholders
+
+  const [restingPlaceholder, setRestingPlaceholder] = useState(() =>
+    pickPlaceholder(sessionId ? followUpPlaceholders : newSessionPlaceholders)
+  )
+
+  const prevSessionIdRef = useRef(sessionId)
+
+  useEffect(() => {
+    const prev = prevSessionIdRef.current
+    prevSessionIdRef.current = sessionId
+
+    if (prev === sessionId) {
+      return
+    }
+
+    // null → id: the new session we're already in just got persisted. Keep the
+    // starter we showed instead of swapping to a follow-up under the user.
+    if (prev == null && sessionId) {
+      return
+    }
+
+    resetBrowseState(prev)
+    setRestingPlaceholder(pickPlaceholder(sessionId ? followUpPlaceholders : newSessionPlaceholders))
+  }, [followUpPlaceholders, newSessionPlaceholders, sessionId])
+
+  // When the transport is disabled it's because the gateway isn't open.
+  // Distinguish a cold start ("Starting Hermes...") from a dropped connection
+  // we're trying to restore. During reconnect, keep the textbox editable so a
+  // flaky network doesn't block drafting; only submit/backend actions stay
+  // disabled until the gateway is open again.
+  return disabled
+    ? reconnecting
+      ? t.composer.placeholderReconnecting
+      : t.composer.placeholderStarting
+    : restingPlaceholder
+}

apps/desktop/src/app/chat/composer/index.tsx

@@ -25,8 +25,7 @@ import {
   browseBackward,
   browseForward,
   deriveUserHistory,
-  isBrowsingHistory,
-  resetBrowseState
+  isBrowsingHistory
 } from '@/store/composer-input-history'
 import {
   $composerPopoutPosition,
@@ -49,7 +48,6 @@ import { AttachmentList } from './attachments'
 import {
   COMPLETION_ACTIONS,
   COMPOSER_FADE_BACKGROUND,
-  pickPlaceholder,
   type QueueEditState,
   slashArgStage,
   slashChipKindForItem,
@@ -66,6 +64,7 @@ import { useComposerDraft } from './hooks/use-composer-draft'
 import { useComposerDrop } from './hooks/use-composer-drop'
 import { useComposerEscCancel } from './hooks/use-composer-esc-cancel'
 import { useComposerMetrics } from './hooks/use-composer-metrics'
+import { useComposerPlaceholder } from './hooks/use-composer-placeholder'
 import { useComposerQueue } from './hooks/use-composer-queue'
 import { useComposerSubmit } from './hooks/use-composer-submit'
 import { useComposerUrlDialog } from './hooks/use-composer-url-dialog'
@@ -186,8 +185,6 @@ export function ChatBar({
 
   const { t } = useI18n()
   const gatewayState = useStore($gatewayState)
-  const newSessionPlaceholders = t.composer.newSessionPlaceholders
-  const followUpPlaceholders = t.composer.followUpPlaceholders
   const reconnecting = gatewayState === 'closed' || gatewayState === 'error'
   const inputDisabled = disabled && !reconnecting
 
@@ -287,45 +284,9 @@ export function ChatBar({
     stashAt
   })
 
-  // Resting placeholder: a starter for brand-new sessions, a continuation for
-  // existing ones. Picked once and only re-rolled when we genuinely move to a
-  // *different* conversation. Critically, the first id assignment of a freshly
-  // started session (null → id, on the first send) is treated as the same
-  // conversation so the placeholder doesn't visibly flip mid-stream.
-  const [restingPlaceholder, setRestingPlaceholder] = useState(() =>
-    pickPlaceholder(sessionId ? followUpPlaceholders : newSessionPlaceholders)
-  )
-
-  const prevSessionIdRef = useRef(sessionId)
-
-  useEffect(() => {
-    const prev = prevSessionIdRef.current
-    prevSessionIdRef.current = sessionId
-
-    if (prev === sessionId) {
-      return
-    }
-
-    // null → id: the new session we're already in just got persisted. Keep the
-    // starter we showed instead of swapping to a follow-up under the user.
-    if (prev == null && sessionId) {
-      return
-    }
-
-    resetBrowseState(prev)
-    setRestingPlaceholder(pickPlaceholder(sessionId ? followUpPlaceholders : newSessionPlaceholders))
-  }, [followUpPlaceholders, newSessionPlaceholders, sessionId])
-
-  // When the transport is disabled it's because the gateway isn't open.
-  // Distinguish a cold start ("Starting Hermes...") from a dropped connection
-  // we're trying to restore. During reconnect, keep the textbox editable so a
-  // flaky network doesn't block drafting; only submit/backend actions stay
-  // disabled until the gateway is open again.
-  const placeholder = disabled
-    ? reconnecting
-      ? t.composer.placeholderReconnecting
-      : t.composer.placeholderStarting
-    : restingPlaceholder
+  // Resting / reconnecting / starting placeholder text, re-rolled only on a real
+  // conversation change.
+  const placeholder = useComposerPlaceholder({ disabled, reconnecting, sessionId })
 
   // Keep the floating box on-screen: re-clamp (with the real measured size +
   // thread bounds) when it pops out and on every window resize — so a position