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

db/persister.go

@@ -8,7 +8,6 @@ import (
 	"github.com/ryanhamamura/c4/db/repository"
 	"github.com/ryanhamamura/c4/game"
 	"github.com/ryanhamamura/c4/snake"
-	"github.com/ryanhamamura/c4/ui"
 )
 
 type GamePersister struct {
@@ -297,7 +296,7 @@ func NewChatPersister(q *repository.Queries) *ChatPersister {
 	return &ChatPersister{queries: q}
 }
 
-func (p *ChatPersister) SaveChatMessage(gameID string, msg ui.C4ChatMessage) error {
+func (p *ChatPersister) SaveChatMessage(gameID string, msg game.ChatMessage) error {
 	return p.queries.CreateChatMessage(context.Background(), repository.CreateChatMessageParams{
 		GameID:    gameID,
 		Nickname:  msg.Nickname,
@@ -307,14 +306,14 @@ func (p *ChatPersister) SaveChatMessage(gameID string, msg ui.C4ChatMessage) err
 	})
 }
 
-func (p *ChatPersister) LoadChatMessages(gameID string) ([]ui.C4ChatMessage, error) {
+func (p *ChatPersister) LoadChatMessages(gameID string) ([]game.ChatMessage, error) {
 	rows, err := p.queries.GetChatMessages(context.Background(), gameID)
 	if err != nil {
 		return nil, err
 	}
-	msgs := make([]ui.C4ChatMessage, len(rows))
+	msgs := make([]game.ChatMessage, len(rows))
 	for i, r := range rows {
-		msgs[i] = ui.C4ChatMessage{
+		msgs[i] = game.ChatMessage{
 			Nickname: r.Nickname,
 			Color:    int(r.Color),
 			Message:  r.Message,