chore: rename quicproquo → quicprochat in Rust workspace

Rename all crate directories, package names, binary names, proto
package/module paths, ALPN strings, env var prefixes, config filenames,
mDNS service names, and plugin ABI symbols from quicproquo/qpq to
quicprochat/qpc.

.gitignore

@@ -16,7 +16,7 @@ data/
 *.convdb-shm
 *.convdb-wal
 *.pending.ks
-qpq-server.toml
+qpc-server.toml
 
 # Internal planning docs (not for public distribution)
 docs/internal/