fix(screenshot): fall back to CDP capture when captureVisibleTab fails (#83)

chrome.tabs.captureVisibleTab reads back the window surface, which fails
outright on some Windows/Chrome combinations (Chromium
FAILURE_REASON_READBACK_FAILED: "Failed to capture tab: image readback
failed"), leaving full-tab screenshots without any working path.

Capture the full tab via CDP Page.captureScreenshot (fromSurface) when
the visible-tab capture rejects; the CDP path goes through the
renderer BeginFrame pipeline instead of the surface readback. When both
paths fail, return cdp_failed with data.reason=screenshot_capture_failed
carrying both underlying messages, and render a CLI hint that points at
the browser-side readback cause.

Refs #71

apps/extension/src/transport/types.ts

@@ -31,7 +31,8 @@ export type RpcErrorReason =
   | "single_select_value_count"
   | "tab_not_active"
   | "restricted_tab_url"
-  | "borrow_conflict";
+  | "borrow_conflict"
+  | "screenshot_capture_failed";
 
 export interface RpcErrorData {
   reason?: RpcErrorReason;