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/ipc/ipc_client.ts

@@ -387,7 +387,7 @@ export class IpcClient {
   public streamMessage(
     prompt: string,
     options: {
-      selectedComponent: ComponentSelection | null;
+      selectedComponents?: ComponentSelection[];
       chatId: number;
       redo?: boolean;
       attachments?: FileAttachment[];
@@ -401,7 +401,7 @@ export class IpcClient {
       chatId,
       redo,
       attachments,
-      selectedComponent,
+      selectedComponents,
       onUpdate,
       onEnd,
       onError,
@@ -441,7 +441,7 @@ export class IpcClient {
               prompt,
               chatId,
               redo,
-              selectedComponent,
+              selectedComponents,
               attachments: fileDataArray,
             })
             .catch((err) => {
@@ -464,7 +464,7 @@ export class IpcClient {
           prompt,
           chatId,
           redo,
-          selectedComponent,
+          selectedComponents,
         })
         .catch((err) => {
           console.error("Error streaming message:", err);