chore: rename project quicnprotochat -> quicproquo (binaries: qpq)

Rename the entire workspace:
- Crate packages: quicnprotochat-{core,proto,server,client,gui,p2p,mobile} -> quicproquo-*
- Binary names: quicnprotochat -> qpq, quicnprotochat-server -> qpq-server,
  quicnprotochat-gui -> qpq-gui
- Default files: *-state.bin -> qpq-state.bin, *-server.toml -> qpq-server.toml,
  *.db -> qpq.db
- Environment variable prefix: QUICNPROTOCHAT_* -> QPQ_*
- App identifier: chat.quicnproto.gui -> chat.quicproquo.gui
- Proto package: quicnprotochat.bench -> quicproquo.bench
- All documentation, Docker, CI, and script references updated

HKDF domain-separation strings and P2P ALPN remain unchanged for
backward compatibility with existing encrypted state and wire protocol.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/quicproquo-server/src/error_codes.rs

@@ -0,0 +1,33 @@
+//! Structured error codes for server RPC responses.
+//!
+//! Every `capnp::Error::failed()` message is prefixed with a stable code
+//! (E001–E020) so clients can match on the code without parsing free-text.
+
+pub const E001_BAD_AUTH_VERSION: &str = "E001";
+pub const E002_EMPTY_TOKEN: &str = "E002";
+pub const E003_INVALID_TOKEN: &str = "E003";
+pub const E004_IDENTITY_KEY_LENGTH: &str = "E004";
+pub const E005_PAYLOAD_EMPTY: &str = "E005";
+pub const E006_PAYLOAD_TOO_LARGE: &str = "E006";
+pub const E007_PACKAGE_EMPTY: &str = "E007";
+pub const E008_PACKAGE_TOO_LARGE: &str = "E008";
+pub const E009_STORAGE_ERROR: &str = "E009";
+pub const E010_OPAQUE_ERROR: &str = "E010";
+pub const E011_USERNAME_EMPTY: &str = "E011";
+pub const E012_WIRE_VERSION: &str = "E012";
+pub const E013_HYBRID_KEY_EMPTY: &str = "E013";
+pub const E014_RATE_LIMITED: &str = "E014";
+pub const E015_QUEUE_FULL: &str = "E015";
+pub const E016_IDENTITY_MISMATCH: &str = "E016";
+pub const E017_SESSION_EXPIRED: &str = "E017";
+pub const E018_USER_EXISTS: &str = "E018";
+pub const E019_NO_PENDING_LOGIN: &str = "E019";
+pub const E020_BAD_PARAMS: &str = "E020";
+pub const E021_CIPHERSUITE_NOT_ALLOWED: &str = "E021";
+pub const E022_CHANNEL_ACCESS_DENIED: &str = "E022";
+pub const E023_CHANNEL_NOT_FOUND: &str = "E023";
+
+/// Build a `capnp::Error::failed()` with the structured code prefix.
+pub fn coded_error(code: &str, msg: impl std::fmt::Display) -> capnp::Error {
+    capnp::Error::failed(format!("{code}: {msg}"))
+}