Implement README TODOs: progress bar, compression level, drag-and-drop

[Copilot]

Three long-standing TODO items in the README were unimplemented. This PR adds all three.

Progress bar

• ZipController.do_subprocess now uses Popen with chunked stdout reading instead of check_call, parsing 7zip's \r-delimited \d+% progress tokens
• A progress_callback: Optional[Callable[[float], None]] parameter threads from compress → start_thread → do_subprocess
• UI wires this via GLib.idle_add for safe cross-thread GTK updates; progress bar resets to 0 on each new compression
• Non-zero exit codes still raise subprocess.CalledProcessError

More compression options

• Added a Level SpinButton (0–9, default 5) to the toolbar; value passed as level to ZipController.compress (the parameter already existed but was hardcoded at the call site)

Drag and drop

• TreeView configured as a URI drop target via drag_dest_set + drag_dest_add_uri_targets
• on_drag_data_received decodes dropped URIs with urllib.parse.urlparse + unquote, deduplicates against file_list, and appends to both the list store and internal state

# zipcontroller.py — progress parsing (simplified)
chunk = process.stdout.read(4096)
for byte in chunk:
    if byte in (b'\r', b'\n'):
        match = re.search(rb'(\d+)%', buf)
        if match:
            progress_callback(int(match.group(1)) / 100.0)
        buf = b''
    else:
        buf += byte

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