feat: add HTMX support for saving filters with HTML response handling

Router changes for saved filters API:
- Add CreateFilterHTML handler to return HTML for HTMX requests
- Extract CreateFilterHTML function to handle filter creation logic
- Add wrapper for POST /api/saved-filters to detect HTMX requests
  - HTMX requests: Return HTML via CreateFilterHTML
  - Regular requests: Return JSON via existing handler
- Add collectFilterFormData helper to gather form data from #filter-form
- Improve error handling for duplicate filter names (409 Conflict)
- HTML responses include error messages for better UX

This enables the save filter modal to work without page refresh,
providing a smoother user experience with immediate visual feedback.

internal/router/filters.go

@@ -1,5 +1,18 @@
 package router
 
+import (
+	"bookhoard/internal/database"
+	"bookhoard/templates"
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"strings"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/labstack/echo/v5"
+)
+
 func registerFiltersRoutes(cfg *Config) {
 	e := cfg.Echo
 
@@ -11,7 +24,118 @@ func registerFiltersRoutes(cfg *Config) {
 	filters := protected.Group("/saved-filters")
 	filters.GET("", cfg.FiltersHandler.GetSavedFilters)
 	filters.GET("/:id", cfg.FiltersHandler.GetSavedFilterByID)
-	filters.POST("", cfg.FiltersHandler.CreateSavedFilter)
+	// POST route with HTMX support
+	filters.POST("", func(c *echo.Context) error {
+		// Check if HTMX request (expects HTML)
+		if c.Request().Header.Get("HX-Request") == "true" {
+			return handleCreateFilterHTML(c, cfg)
+		}
+		// Otherwise return JSON (API clients)
+		return cfg.FiltersHandler.CreateSavedFilter(c)
+	})
 	filters.PUT("/:id", cfg.FiltersHandler.UpdateSavedFilter)
 	filters.DELETE("/:id", cfg.FiltersHandler.DeleteSavedFilter)
 }
+
+// handleCreateFilterHTML renders HTML response for HTMX requests
+func handleCreateFilterHTML(c *echo.Context, cfg *Config) error {
+	user, ok := c.Get("user").(database.Users)
+	if !ok {
+		return c.String(http.StatusUnauthorized, "Unauthorized")
+	}
+	userUUID := uuid.UUID(user.ID.Bytes)
+	// Parse form data
+	var req struct {
+		Name         string `form:"filter_name"`
+		ResourceType string `form:"resource_type"`
+	}
+
+	if err := c.Bind(&req); err != nil {
+		return c.String(http.StatusBadRequest, "Invalid request body")
+	}
+	if req.Name == "" || req.ResourceType == "" {
+		return c.String(http.StatusBadRequest, "name and resource_type are required")
+	}
+	// Collect filter data from individual form fields (via hx-include)
+	filtersMap := collectFilterFormData(c)
+	// Convert to JSON for database storage
+	filtersJSON, err := json.Marshal(filtersMap)
+	if err != nil {
+		return c.String(http.StatusInternalServerError, "Failed to encode filters")
+	}
+	// Create filter using service layer
+	filter, err := cfg.Queries.CreateSavedFilter(c.Request().Context(), database.CreateSavedFilterParams{
+		UserID:       pgtype.UUID{Bytes: userUUID, Valid: true},
+		Name:         req.Name,
+		ResourceType: req.ResourceType,
+		Filters:      filtersJSON,
+	})
+	if err != nil {
+		errorMsg := err.Error()
+		// Check for duplicate name error (wrapped or unwrapped)
+		if strings.Contains(errorMsg, "filter with name") && strings.Contains(errorMsg, "already exists") {
+			if c.Request().Header.Get("HX-Request") == "true" {
+				return c.HTML(http.StatusConflict, `<div hx-swap-oob="innerHTML:#filter-save-error" style="color: #ef4444; padding: 0.75rem; border-radius: 0.5rem; margin-bottom: 1rem;">⚠️ Filter with this name already exists</div>`)
+			}
+			return c.String(http.StatusConflict, "Filter with this name already exists")
+		}
+		return c.String(http.StatusInternalServerError, "failed to create filter: "+errorMsg)
+	}
+	// Render filter item HTML to buffer
+	var buf bytes.Buffer
+	filterID := uuid.Must(uuid.FromBytes(filter.ID.Bytes[:]))
+	err = templates.FilterItem(
+		filterID.String(),
+		filter.Name,
+	).Render(c.Request().Context(), &buf)
+	if err != nil {
+		return c.HTML(http.StatusInternalServerError, `<div style="color: red;">Render error</div>`)
+	}
+
+	return c.HTML(http.StatusCreated, buf.String())
+}
+
+// collectFilterFormData extracts filter values from form fields
+func collectFilterFormData(c *echo.Context) map[string]string {
+	filters := make(map[string]string)
+
+	// Collect individual form fields
+	if q := c.FormValue("q"); q != "" {
+		filters["q"] = q
+	}
+	if author := c.FormValue("author_filter"); author != "" {
+		filters["author_filter"] = author
+	}
+	if tags := c.FormValue("tags_filter"); tags != "" {
+		filters["tags_filter"] = tags
+	}
+	if series := c.FormValue("series_filter"); series != "" {
+		filters["series_filter"] = series
+	}
+	if language := c.FormValue("language_filter"); language != "" {
+		filters["language_filter"] = language
+	}
+	if yearMin := c.FormValue("year_min"); yearMin != "" {
+		filters["year_min"] = yearMin
+	}
+	if yearMax := c.FormValue("year_max"); yearMax != "" {
+		filters["year_max"] = yearMax
+	}
+	if hasCover := c.FormValue("has_cover"); hasCover != "" {
+		filters["has_cover"] = hasCover
+	}
+	if libraryID := c.FormValue("library_id"); libraryID != "" {
+		filters["library_id"] = libraryID
+	}
+	if sort := c.FormValue("sort"); sort != "" {
+		filters["sort"] = sort
+	}
+	if limit := c.FormValue("limit"); limit != "" {
+		filters["limit"] = limit
+	}
+	if offset := c.FormValue("offset"); offset != "" {
+		filters["offset"] = offset
+	}
+
+	return filters
+}