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).
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