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.

native-host/kget_capture_host.py

@@ -3,6 +3,8 @@
 
 import json
 import os
+import re
+import shutil
 import struct
 import subprocess
 import sys
@@ -62,14 +64,87 @@ def add_transfer(url, filename):
     try:
         result = subprocess.run(cmd, capture_output=True, text=True, timeout=15)
     except FileNotFoundError:
-        return False, "dbus-send not found on this system"
+        return False, "dbus-send not found on this system", None, dest
     except subprocess.TimeoutExpired:
-        return False, "dbus-send call timed out (is KGet running?)"
+        return False, "dbus-send call timed out (is KGet running?)", None, dest
 
     if result.returncode != 0:
         stderr = result.stderr.strip()
-        return False, f"dbus-send failed: {stderr or 'unknown error'}"
-    return True, None
+        return False, f"dbus-send failed: {stderr or 'unknown error'}", None, dest
+
+    # addTransfer replies with an array of the new transfer's D-Bus object
+    # path(s), e.g. `array [ string "/KGet/Transfers/9" ]` -- grab the first
+    # one so we can poll it for completion (see spawn_notifier).
+    match = re.search(r'string "(/KGet/Transfers/\d+)"', result.stdout)
+    transfer_path = match.group(1) if match else None
+    return True, None, transfer_path, dest
+
+
+# KGet only fires its own "Download Finished" desktop notification if the
+# transfer was still in a non-Finished state the first time KGet's UI
+# observed it (see GenericObserver::transfersChangedEvent in KGet's
+# core/kget.cpp: `status() == Finished && startStatus() != Finished`).
+# Small/fast downloads routinely finish before that first observation, so
+# KGet silently skips the notification for them -- confirmed by tracing
+# actual D-Bus traffic against a running KGet instance. We replace it with
+# our own, driven by polling the transfer's real progress, so captures
+# always get a completion notification regardless of how fast they finish.
+_NOTIFIER_SCRIPT = """
+import subprocess, sys, time
+
+path, dest = sys.argv[1], sys.argv[2]
+
+def get_percent():
+    try:
+        result = subprocess.run(
+            ["dbus-send", "--session", "--print-reply", "--dest=org.kde.kget",
+             path, "org.kde.kget.transfer.percent"],
+            capture_output=True, text=True, timeout=10,
+        )
+    except (subprocess.TimeoutExpired, FileNotFoundError):
+        return None
+    if result.returncode != 0:
+        return None
+    for line in result.stdout.splitlines():
+        line = line.strip()
+        if line.startswith("int32"):
+            return int(line.split()[-1])
+    return None
+
+deadline = time.time() + 6 * 3600
+finished = False
+while time.time() < deadline:
+    percent = get_percent()
+    if percent is None:
+        break  # transfer object is gone: finished+cleared, cancelled, or KGet restarted
+    if percent >= 100:
+        finished = True
+        break
+    time.sleep(2)
+
+if finished:
+    subprocess.run(
+        ["notify-send", "--app-name=KGet Capture", "--icon=kget",
+         "Download complete", dest.rsplit("/", 1)[-1]],
+        timeout=10,
+    )
+"""
+
+
+def spawn_notifier(transfer_path, dest):
+    if not transfer_path or not shutil.which("notify-send"):
+        return
+    # Detached (new session, no inherited pipes) so it keeps running after
+    # Chrome closes the native-messaging connection and kills this process --
+    # sendNativeMessage() tears down its native host as soon as it gets a
+    # reply, which is long before a real download finishes.
+    subprocess.Popen(
+        [sys.executable, "-c", _NOTIFIER_SCRIPT, transfer_path, dest],
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+        start_new_session=True,
+    )
 
 
 def main():
@@ -78,7 +153,9 @@ def main():
         write_message({"ok": False, "error": "invalid request"})
         return
 
-    ok, error = add_transfer(message["url"], message.get("filename"))
+    ok, error, transfer_path, dest = add_transfer(message["url"], message.get("filename"))
+    if ok:
+        spawn_notifier(transfer_path, dest)
     write_message({"ok": True} if ok else {"ok": False, "error": error})