refactor: replace via framework with chi + templ + datastar

Migrate from the via meta-framework to direct dependencies:
- chi for routing, templ for HTML templates, datastar for SSE/reactivity
- Feature-sliced architecture (features/{auth,lobby,c4game,snakegame}/)
- Shared layouts and components (features/common/)
- Handler factory pattern (HandleX(deps) http.HandlerFunc)
- Embedded NATS server (nats/), SCS sessions (sessions/), chi router wiring (router/)
- Move ChatMessage domain type from ui package to game package
- Remove old ui/ package (gomponents-based via/h views)
- Remove via dependency from go.mod entirely

game/types.go

@@ -67,3 +67,11 @@ func (g *Game) WinningCellsFromJSON(data string) error {
 	}
 	return json.Unmarshal([]byte(data), &g.WinningCells)
 }
+
+// ChatMessage is the domain type for persisted C4 chat messages.
+type ChatMessage struct {
+	Nickname string `json:"nickname"`
+	Color    int    `json:"color"` // 1=Red, 2=Yellow
+	Message  string `json:"message"`
+	Time     int64  `json:"time"`
+}