refactor(desktop): colocate hook/component families into scoped folders

Single-scoped helpers/sub-files were sitting flat in shared/grab-bag dirs.
Fold each family into its own folder (index = the export, dir resolution keeps
public import paths intact), dropping the now-redundant filename prefix:

- session/hooks/use-prompt-actions.ts (+ -utils, + tests)
  -> use-prompt-actions/{index,utils}.ts (+ tests)
- components/assistant-ui/thread* + assistant/system/user message renderers
  -> assistant-ui/thread/{index,content,status,message-parts,timestamp,types,
     list,timeline,timeline-data,assistant-message,system-message,user-message,
     user-edit-composer,user-message-text} (+ tests)
- components/assistant-ui/tool-fallback(+model)/tool-approval
  -> assistant-ui/tool/{fallback,fallback-model,approval} (+ tests)

Pure move + import rewrites; no behaviour change. App-wide shared primitives
(markdown-text, directive-text, tooltip-icon-button, clarify-tool, ansi-text,
message-render-boundary) stay flat. desktop-controller intentionally left in
app/ (route root; foldering would churn ~80 relative imports for no gain).

apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx

@@ -8,7 +8,7 @@ import { $composerAttachments, type ComposerAttachment } from '@/store/composer'
 import { $busy, $connection, $messages, $sessions, setSessions } from '@/store/session'
 import type { SessionInfo } from '@/types/hermes'
 
-import { uploadComposerAttachment, usePromptActions } from './use-prompt-actions'
+import { uploadComposerAttachment, usePromptActions } from '.'
 
 vi.mock('@/hermes', () => ({
   getProfiles: vi.fn(async () => ({ profiles: [] })),

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

@@ -72,7 +72,7 @@ import type {
   SessionSteerResponse,
   SessionTitleResponse,
   SlashExecResponse
-} from '../../types'
+} from '../../../types'
 
 import {
   _submitInFlight,
@@ -93,7 +93,7 @@ import {
   visibleUserIndexAtOrdinal,
   visibleUserOrdinal,
   withSessionBusyRetry
-} from './use-prompt-actions-utils'
+} from './utils'
 
 interface HandoffResult {
   ok: boolean

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

@@ -15,7 +15,7 @@ import {
   slashStatusText,
   visibleUserIndexAtOrdinal,
   visibleUserOrdinal
-} from './use-prompt-actions-utils'
+} from './utils'
 
 describe('isSessionIdCandidate', () => {
   it('accepts the timestamped and hex id forms', () => {