Fix Chrome save-as prompt and unreliable finish notifications

Chrome's own "Ask where to save each file" setting shows a native
picker before the extension can ever intercept the download, and no
extension API can read or suppress it -- surface this clearly in
install.sh, the README, and a popup shortcut instead of leaving it
buried as a footnote.

Also fixes silently missing "download complete" notifications: KGet
only fires its own finished-notification if a transfer was observed
in a non-finished state at least once, so small/fast downloads that
complete before that first observation never get one. The native
host now drives its own notification via notify-send, polling actual
transfer progress instead of relying on KGet's internal check, and
install.sh disables KGet's own notification to avoid duplicates.

extension/popup.js

@@ -14,3 +14,7 @@ checkbox.addEventListener("change", () => {
   chrome.storage.local.set({ autoCaptureEnabled: checkbox.checked });
   renderStatus(checkbox.checked);
 });
+
+document.getElementById("open-download-settings").addEventListener("click", () => {
+  chrome.tabs.create({ url: "chrome://settings/downloads" });
+});