Fix M10: consolidate .url Internet Shortcut parsing into shared utility

Both DownloadBar (renderer, drag-drop) and deeplink.ts (main, argv) had their
own regex to extract URL= from a Windows Internet Shortcut file -- with subtly
different patterns (/^\s*URL\s*=\s*(\S+)/ vs /^URL=(.+)$/).

Added parseUrlShortcutContent() to @shared/ipc. Both callers now delegate to
it and apply their own http/https guard (looksLikeUrl in renderer, asHttpUrl
in main). The local parseUrlFile wrapper in DownloadBar is kept as the
validation compositing point; it's now a one-liner.

typecheck + 242 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/shared/ipc.ts

@@ -771,6 +771,16 @@ export interface TerminalRunResult {
   error?: string
 }
 
+/**
+ * Extract the `URL=` value from a Windows Internet Shortcut (.url) file's content.
+ * Returns the raw URL string (untrimmed), or null if not found.
+ * Callers validate the result with their own http/https check.
+ */
+export function parseUrlShortcutContent(text: string): string | null {
+  const m = /^\s*URL\s*=\s*(\S+)/im.exec(text)
+  return m?.[1]?.trim() ?? null
+}
+
 /** Overall queue state for the Windows taskbar progress bar (Phase O). */
 export interface TaskbarProgress {
   /** 0..1 overall fraction; ignored when mode is 'none' */