[Visual bug] Export UI shows stale "Export Complete" notification during re-export

[VintaGist]

Describe the bug
When re-exporting the same video in a different format, the export UI doesn't reset and shows conflicting status messages. It displays "Export Complete" and "Video saved successfully!" from the first export while simultaneously showing "Processing" status and active progress bar for the second export.

To Reproduce
Steps to reproduce the behavior:

1. Record a video
2. Export it in one format (e.g., MP4) - wait for completion
3. Export the same video again in a different format (e.g., WebM)
4. Observe that the UI shows "Export Complete" / "Video saved successfully!" at the top while the progress bar shows current progress and status shows "Processing"

Expected behavior
When starting a re-export of the same recording, the UI should completely reset:

• Clear previous "Export Complete" notification
• Clear "Video saved successfully!" message
• Show only current export status ("Exporting Video" / "This may take a moment...")
• Display current progress bar starting from 0%

Screenshots
First export (completed):

Second export of the same video (in progress, showing stale UI):

Notice: The second export shows both "Export Complete" (from first export) AND "Processing" (current export) simultaneously, creating confusion about the actual export state.

Desktop (please complete the following information):

• OS: Ubuntu 25.10 x86_64
• Wayland (Mutter)
• DE: GNOME 49.0
• App Version: v1.0.2 (AppImage)

Additional context
This is a UI state management issue where success notifications from previous exports aren't cleared when re-exporting the same recording. Users might think the export is already complete when it's actually still processing, potentially leading to premature dialog closure or confusion about export status.