Fix TUI startup and test script bugs

- TUI: replace per-call bufio.Scanner with a goroutine+channel reader
  (lineReader) so a single scanner lives for the connection lifetime;
  previous pattern silently dropped messages due to scanner read-ahead
- test-tui.sh / test-network.sh: add `return 0` to wait_port — when the
  port opens and the while condition becomes false, bash returned the
  condition's exit code (1) to the caller, tripping set -e immediately
- .gitignore: use /tui (root-only) instead of tui to avoid ignoring cmd/tui/

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

.gitignore

@@ -10,4 +10,4 @@
 .env
 .DS_Store
 *.swp
-tui
+/tui