feat: Phase 1 - TUI Foundation with Textual

- Added Textual TUI framework with modern terminal UI
- Created main TUI app with navigation, message view, event log, state inspector
- Added comprehensive CSS styling for Discord-like appearance
- Implemented navigation pane, message area, event log, state tree, command bar
- Added keyboard shortcuts (Ctrl+Q quit, Ctrl+C command, Ctrl+N/P nav, etc.)
- Integrated TUI with existing REST/Gateway servers and state management
- Added CLI flag --tui/--no-tui to launch TUI mode
- Added Textual and textual-dev to optional dependencies

All 19 tests passing.

pyproject.toml

@@ -35,6 +35,10 @@ dev = [
     "ruff>=0.1.0",
     "mypy>=1.5.0",
 ]
+tui = [
+    "textual>=0.52.0",
+    "textual-dev>=0.52.0",
+]
 
 [project.scripts]
 apicord = "apicord.main:main"