feat: multi-component-selector (#1728)

<!-- This is an auto-generated description by cubic. -->
## Summary by cubic
Adds multi-component selection in the preview and sends all selected
components to chat for targeted edits. Updates overlays, UI, and IPC to
support arrays, smarter context focusing, and cross-platform path
normalization.

- **New Features**
- Select multiple components in the iframe; selection mode stays active
until you deactivate it.
- Show a scrollable list of selections with remove buttons and a Clear
all; remove from the list or click an overlay in the preview to
deselect. Sending clears all overlays.
- Separate hover vs selected overlays with labels on hover; overlays
persist after deactivation and re-position on layout changes/resizes.
- Chat input and streaming now send selectedComponents; server builds
per-component snippets and focuses their files in smart context.

- **Migration**
- Replace selectedComponentPreviewAtom with
selectedComponentsPreviewAtom (ComponentSelection[]).
- ChatStreamParams now uses selectedComponents; migrate any
single-selection usages.
  - previewIframeRefAtom added for clearing overlays from the parent.

<sup>Written for commit da0d64cc9e9f83fbf4b975278f6c869f0d3a8c7d.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

src/hooks/useStreamChat.ts

@@ -69,14 +69,14 @@ export function useStreamChat({
       chatId,
       redo,
       attachments,
-      selectedComponent,
+      selectedComponents,
       onSettled,
     }: {
       prompt: string;
       chatId: number;
       redo?: boolean;
       attachments?: FileAttachment[];
-      selectedComponent?: ComponentSelection | null;
+      selectedComponents?: ComponentSelection[];
       onSettled?: () => void;
     }) => {
       if (
@@ -106,7 +106,7 @@ export function useStreamChat({
       let hasIncrementedStreamCount = false;
       try {
         IpcClient.getInstance().streamMessage(prompt, {
-          selectedComponent: selectedComponent ?? null,
+          selectedComponents: selectedComponents ?? [],
           chatId,
           redo,
           attachments,