Redoubt 2.1.0

Improvements

Settings menu: New top-left ☰ Settings menu (also F2) with About, Show card, Contact list and Add contact.
Contact management in the UI: Add contacts through a validated form, browse saved contacts with name, IP, public key and fingerprint, and revoke them from the contact list.
Revocation with confirmation: Revoking a contact deletes its messages and outbox, closes any live session, wipes the Double Ratchet state, drops pending ACKs/presence, and prevents in-flight handshakes from installing a new session.
Shared contact validation: contact_validation.py centralizes name/IP/public-key/fingerprint checks for both the UI form and the CLI.
Legacy CLI retained: --add-contact, --list-contacts and --remove-contact are still available and documented.
About screen: Shows author, version and license.
Test coverage: Added tests for the settings menu, theme cycling, settings persistence and revoked-contact session handling.

Revocation is enforced at runtime: _install_session refuses sessions for contacts no longer present in storage, and drop_contact() removes live session/ratchet state and pending retransmissions.
Contact validation (fingerprint must match the public key, own identity rejected, duplicate contacts rejected) is now shared between CLI and UI, avoiding divergent checks.
Wire protocol and message cryptography are unchanged.

Restored the missing packaging/redoubt.desktop, so AppImage builds ship a valid desktop entry again (v2.0.0 packaging regression).
Corrected the desktop entry Version field (it is the Desktop Entry specification version 1.0, not the Redoubt version).
Documentation updates: settings menu, legacy CLI contact commands, module table, revocation behavior and AppImage output path.

Version 2.1.0 clients ARE compatible with version 2.0.0 clients (but not 1.x.x): the wire protocol and message cryptography are unchanged.
Existing identity keys, contacts and locally stored history remain valid.
The legacy CLI contact commands remain available.