Implement networking, RSA authentication, and persistent chat

[LordofGhost]

Summary

This PR implements the complete client-server communication flow for the messenger. It adds a Qt/QML user interface, RSA-based authentication, user management, and persistent chat history using SQLite.

Changes

• Implement TCP communication between the client and server
• Add a shared, versioned messaging protocol
• Implement RSA-based challenge-response authentication
• Add registration requests that can be approved or rejected through the server CLI
• Add local RSA key management and storage to the client
• Persist users and chat messages using SQLite
• Load and distribute previous chat history
• Broadcast new messages to authenticated clients
• Add server commands for:
  • Listing, approving, and rejecting registration requests
  • Listing and deleting users
  • Clearing the chat history
• Add a Qt Quick UI for connections, key management, and chatting
• Expand the README with feature details and screenshots
• Document the connection and authentication flow
• Update the RSA submodule

Security

• Private RSA keys remain exclusively on the client.
• Each authentication attempt uses fresh client and server nonces.
• Messages and chat history are only available after successful authentication.
• Invalid protocol versions, message types, and signatures are rejected.
• Authentication attempts have a timeout.

[Jochengehtab]

Please run clang format before merging

[Copilot]

Pull request overview

This PR introduces an end-to-end client/server messenger implementation: a shared versioned protocol, TCP networking, RSA-based challenge/response authentication, and SQLite-backed persistence, plus a Qt Quick/QML client UI and updated documentation.

Changes:

• Adds a shared protocol (Message, message types, limits, transcript + RNG helpers) used by both client and server.
• Implements server-side session handling, authentication flow, registration request workflow, message broadcast, and SQLite storage/migrations.
• Implements a Qt Quick client UI with connection/key management plus client-side authentication and message send/receive logic.

Reviewed changes

Copilot reviewed 23 out of 26 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/shared/message.h	Defines protocol constants, message types, Message struct, transcript + RNG helpers.
src/shared/message.cpp	Implements QDataStream serialization, transcript encoding, secure random bytes helper.
src/shared/CMakeLists.txt	Adds Messenger-Shared static library and exports includes/Qt dependency.
src/server/Storage/message_store.h	Declares SQLite-backed storage API for users, registration requests, and messages.
src/server/Storage/message_store.cpp	Implements SQLite open/migrations, registration request/user management, message save/load.
src/server/session.h	Declares per-connection session state, authentication context, and socket IO signals/slots.
src/server/session.cpp	Implements message IO framing via QDataStream transactions + authentication timeout handling.
src/server/server.h	Expands server into a QObject with QTcpServer, active sessions, and MessageStore.
src/server/server.cpp	Implements listen/shutdown, auth handshake, registration request handling, broadcast + admin CLI.
src/server/Migrations/001_create_messages.sql	Adds initial schema for users and messages + indexes.
src/server/Migrations/002_create_registration_requests.sql	Adds schema for registration requests + indexes/constraints.
src/server/CMakeLists.txt	Links Qt Core/Network/Sql, shared protocol lib + RSA lib; copies migrations post-build.
src/client/network_manager.h	Declares QML-facing networking/authentication state machine and signals.
src/client/network_manager.cpp	Implements TCP connect/auth, challenge signing, protocol parsing, and chat message send/receive.
src/client/connection_store.h	Declares persistent connection/key storage and background RSA key generation controls.
src/client/connection_store.cpp	Implements key management on disk, settings persistence, and async key generation workflow.
src/client/CMakeLists.txt	Links QuickControls2 + shared protocol + RSA, adds new client sources.
src/client/client.h	Adds owned NetworkManager and ConnectionStore for QML context.
src/client/client.cpp	Wires up QML context properties, sets app/org names, sets Quick style.
src/client/Main.qml	Implements full UI for connect/auth, RSA key management, and chat interface.
docs/connection-establishment.md	Documents handshake, message types, transcript definition, and server/client behavior.
README.md	Updates product/architecture overview and references authentication flow documentation.
.gitignore	Ignores SQLite artifacts and IDE settings directory.

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