frontend: add admin settings page for base URL configuration

Phase 2: Create admin UI for system configuration

- Add new /admin/settings route in frontend.go (protected by AdminMiddleware)
- Create admin_settings.templ with HTMX-powered form for base URL
- Add Settings link to admin sidebar navigation
- Admin settings form submits via HTMX to PUT /api/system/config
- Success message displays after save with updated form

The settings page allows admins to configure the base URL used for
device sync URLs, OPDS endpoints, and API access.

internal/router/frontend.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"time"
 
+	"bookhoard/internal/config"
 	"bookhoard/internal/database"
 	"bookhoard/internal/handlers"
 	"bookhoard/internal/services"
@@ -530,8 +531,14 @@ func registerFrontendRoutes(cfg *Config) {
 			pendingList = convertPending(pendingMaps)
 		}
 
+		// Get base URL from database config with fallback to config/env var
+		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
+		if baseURL == "" {
+			baseURL = cfg.Cfg.BaseURL
+		}
+
 		var buf bytes.Buffer
-		err = templates.Devices(user, devices, pendingList, errorMsg, cfg.Cfg.BaseURL).Render(c.Request().Context(), &buf)
+		err = templates.Devices(user, devices, pendingList, errorMsg, baseURL).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}
@@ -723,6 +730,30 @@ func registerFrontendRoutes(cfg *Config) {
 	}))
 
 	// Admin: Get profile modal for editing user
+	frontendProtected.GET("/admin/settings", handlers.AdminMiddleware(func(c *echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+
+		// Fetch current system configuration - just base_url
+		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
+		if baseURL == "" {
+			baseURL = cfg.Cfg.BaseURL
+		}
+
+		systemConfig := map[string]string{
+			"base_url": baseURL,
+		}
+
+		var buf bytes.Buffer
+		err = templates.AdminSettings(user, systemConfig, "").Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	}))
+
 	frontendProtected.GET("/admin/users/:id/profile-modal", handlers.AdminMiddleware(func(c *echo.Context) error {
 		// Get target user ID from URL
 		targetUserID := c.Param("id")
@@ -790,8 +821,14 @@ func registerFrontendRoutes(cfg *Config) {
 			pendingList = convertPending(pendingMaps)
 		}
 
+		// Get base URL from database config with fallback to config/env var
+		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
+		if baseURL == "" {
+			baseURL = cfg.Cfg.BaseURL
+		}
+
 		var buf bytes.Buffer
-		err = templates.Devices(user, devices, pendingList, errorMsg, cfg.Cfg.BaseURL).Render(c.Request().Context(), &buf)
+		err = templates.Devices(user, devices, pendingList, errorMsg, baseURL).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}