feat(dashboard): implement Phase 10.5 Custom Section Builder

Phase 10.5.1: Add /custom-section frontend route
- Added route handler in internal/router/frontend.go
- Fetches user libraries and renders custom section builder template

Phase 10.5.2: Create custom section builder template
- Created templates/custom_section.templ with full UI
- Includes section details form, filter rules builder, manual book selection
- Live preview functionality with preview container
- Form actions for save/cancel

Phase 10.5.3: Create custom-section-builder TypeScript
- Created web/src/custom-section-builder.ts with 13+ filter fields
- Filter fields: title, author, genre, series, progress, rating, date_added, last_read, publisher, language, format, tags, narrators
- Procedural/imperative style (no OOP) as per guidelines
- Rule builder with AND/OR logic support
- Book search and multi-select functionality
- Live preview via /api/collections/preview endpoint
- Form validation and submission to /api/collections

Phase 10.5.4: Build TypeScript modules
- Compiled custom-section-builder.ts to web/static/custom-section-builder.js
- Verified successful compilation with no errors
- All existing TypeScript modules continue to compile

Phase 10.5.5: Add Bruno tests for custom section creation
- create-custom-section-rules.bru: Test creating section with filter rules
- create-custom-section-manual.bru: Test creating section with manual book selection
- create-custom-section-missing-fields.bru: Test error handling for missing required fields

Phase 10.6: Build Verification
- ✅ TypeScript modules compile successfully
- ✅ Templates generate successfully
- ✅ Go build succeeds with no compilation errors
- ✅ All build artifacts verified (dashboard.js, custom-section-builder.js, dashboard_templ.go, custom_section_templ.go)

This completes the Custom Section Builder feature, allowing users to create
personalized dashboard sections with flexible filter rules or manual book selection.

internal/router/frontend.go

@@ -190,6 +190,38 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	})
 
+	// Custom Section Builder page
+	frontendProtected.GET("/custom-section", func(c echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return c.HTML(http.StatusInternalServerError, "Error loading user")
+		}
+
+		userUUID, _ := uuid.Parse(user.ID)
+		libraries, err := cfg.Queries.GetUserVisibleLibraries(c.Request().Context(), uuidToPGType(userUUID))
+		if err != nil {
+			return c.HTML(http.StatusInternalServerError, "Error loading libraries")
+		}
+
+		libData := make([]templates.LibraryData, len(libraries))
+		for i, lib := range libraries {
+			libUUID, _ := uuid.FromBytes(lib.ID.Bytes[0:16])
+			libData[i] = templates.LibraryData{
+				ID:          libUUID.String(),
+				Name:        lib.Name,
+				Description: getText(lib.Description),
+				TypeName:    lib.TypeName,
+			}
+		}
+
+		var buf bytes.Buffer
+		err = templates.CustomSectionBuilder(user, libData).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
 	// Progress page
 	frontendProtected.GET("/progress", func(c echo.Context) error {
 		user, err := getTemplateUserWithTheme(c, cfg)