feat: add router package structure for route organization

Create internal/router/ package to organize route registration:
- router.go: Main router setup and configuration
- auth.go: Authentication routes (login, register, profile, etc.)
- docs.go: Documentation routes
- frontend.go: Frontend SSR routes (/, /login, /admin, etc.)
- helpers.go: Helper functions for template rendering

This is the first step in refactoring 858-line main.go into
a more maintainable structure following Go best practices.

Routes themselves have NOT changed - only organization.

internal/router/helpers.go

@@ -0,0 +1,97 @@
+package router
+
+import (
+	"context"
+	"time"
+
+	"bookhoard/internal/handlers"
+	"bookhoard/templates"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/labstack/echo/v4"
+)
+
+func getTemplateUserWithTheme(c echo.Context, cfg *Config) (templates.User, error) {
+	userID := c.Get("user_id").(string)
+	userEmail := c.Get("user_email").(string)
+	userUsername := c.Get("user_username").(string)
+	userRole := c.Get("user_role").(string)
+
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return templates.User{}, err
+	}
+
+	userDB, err := cfg.Queries.GetUser(c.Request().Context(), uuidToPGType(userUUID))
+	if err != nil {
+		return templates.User{}, err
+	}
+
+	userTheme := "tokyo-night"
+	if userDB.Theme.Valid {
+		userTheme = userDB.Theme.String
+	}
+
+	return templates.User{
+		ID:       userID,
+		Email:    userEmail,
+		Username: userUsername,
+		Role:     userRole,
+		Theme:    userTheme,
+	}, nil
+}
+
+func convertDevices(deviceInfos []handlers.DeviceInfo) []templates.DeviceData {
+	result := make([]templates.DeviceData, len(deviceInfos))
+	for i, d := range deviceInfos {
+		lastSync := ""
+		if d.LastSync != nil {
+			lastSync = d.LastSync.Format(time.RFC3339)
+		}
+		lastSeen := ""
+		if d.LastSeen != nil {
+			lastSeen = d.LastSeen.Format(time.RFC3339)
+		}
+		result[i] = templates.DeviceData{
+			ID:          d.ID.String(),
+			DeviceName:  d.DeviceName,
+			DeviceType:  d.DeviceType,
+			SyncEnabled: d.SyncEnabled,
+			LastSync:    lastSync,
+			LastSeen:    lastSeen,
+		}
+	}
+	return result
+}
+
+func convertPending(pending []map[string]interface{}) []templates.PendingRegistrationData {
+	result := make([]templates.PendingRegistrationData, len(pending))
+	for i, p := range pending {
+		result[i] = templates.PendingRegistrationData{
+			RegistrationID: p["registration_id"].(string),
+			DeviceName:     p["device_name"].(string),
+			DeviceType:     p["device_type"].(string),
+			ExpiresAt:      p["expires_at"].(string),
+		}
+	}
+	return result
+}
+
+func pingDB(cfg *Config, ctx context.Context) error {
+	if cfg.DBPool != nil {
+		if pool, ok := cfg.DBPool.(*pgxpool.Pool); ok {
+			return pool.Ping(ctx)
+		}
+	}
+	return nil
+}
+
+func parseUUID(s string) (uuid.UUID, error) {
+	return uuid.Parse(s)
+}
+
+func uuidToPGType(u uuid.UUID) pgtype.UUID {
+	return pgtype.UUID{Bytes: [16]byte(u), Valid: true}
+}