Secure Intercom Ubuntu Beta R3 — invitation deletion

From the desktop account that runs Secure Intercom, close the app and run:
    sh install.sh
Do not use sudo. This updates the current user's application in
~/.local/opt/SecureIntercom. Existing activation, identity, contacts and audio
settings live separately and are retained. The previous application is backed up.
The installer checks the existing R2/R3 files before replacement.

Requires the Ubuntu invitation-ID cancellation extension on the server.
The previously deployed Windows 0.6.5 cancellation endpoint alone is insufficient
for older Ubuntu history, which retained IDs rather than original codes.

Open Saved invitations, select an entry and choose Delete invitation.
Confirm deletion. An unused code becomes unusable; an accepted invitation loses
only its history entry, leaving existing contacts intact. Other invitations stay.
The window shows all saved history, not only the three-row main-screen summary.
Deletion requires connectivity and an active enrolled identity. It is disabled
during calls and while a deletion is pending. On failure, the entry is retained;
retry after restoring connectivity. Retrying a completed server cancellation is safe.

R2 keyring preflight and repair behavior is preserved. Audio/ringer implementation
and ringtone are unchanged. Sidetone control remains absent as in R2.
Required runtime: Ubuntu 24.04 x86_64, python3-tk, secret-tool (libsecret-tools),
timeout, pw-play, arecord and aplay. Install dependencies through Ubuntu normally.

Acceptance after installation:
- Existing activation and contacts remain; title says Ubuntu beta R3.
- Create two disposable invitations. Delete one, retain the other, restart app.
- Try the deleted code on Windows: rejected. Remaining invitation still works.
- Delete an invitation associated with an existing contact: contact remains.
- Confirm calls both ways, incoming ringing, answer stops ringing, headset speech.
- Check failure feedback if disconnected; reconnect and retry.
This package must pass actual desktop acceptance before public release.
