feat(handlers): add file icons with standard and retro variants

Introduce file icon support to the file browser. Icons are loaded on
startup and mapped based on file name and content type.

- Load file icon mappings in the App handler initialization.
- Add `HasThumbnail`, `IconURL`, and `IconRetroURL` to the file view.
- Update CSS to support displaying file icons alongside thumbnails.
- Add retro theme support to swap standard icons with pixelated retro
  variants when the retro theme is active.

backend/libs/handlers/icons.go

@@ -0,0 +1,152 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// fileIcon holds the two icon filenames for a file type: the standard (modern)
+// icon and the retro (Win98) icon. The filenames are resolved against
+// static/file-icons/standard and static/file-icons/retro respectively.
+type fileIcon struct {
+	Standard string `json:"standard"`
+	Retro    string `json:"retro"`
+}
+
+type iconType struct {
+	Mime       string   `json:"mime"`
+	Standard   string   `json:"standard"`
+	Retro      string   `json:"retro"`
+	Extensions []string `json:"extensions"`
+}
+
+type iconMapFile struct {
+	Default iconType   `json:"default"`
+	Types   []iconType `json:"types"`
+}
+
+type mimeRule struct {
+	pattern string // exact mime ("application/pdf") or major prefix ("image/")
+	prefix  bool
+	icon    fileIcon
+}
+
+// fileIconSet is the loaded icon map: an extension lookup plus content-type
+// rules and a fallback. It is built once at startup from icon-map.json.
+type fileIconSet struct {
+	byExt    map[string]fileIcon
+	byMime   []mimeRule
+	fallback fileIcon
+}
+
+// loadFileIcons reads static/file-icons/icon-map.json and indexes it by
+// extension and content type so icons can be assigned at render time.
+func loadFileIcons(staticDir string) (*fileIconSet, error) {
+	data, err := os.ReadFile(filepath.Join(staticDir, "file-icons", "icon-map.json"))
+	if err != nil {
+		return nil, err
+	}
+	var raw iconMapFile
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return nil, err
+	}
+
+	set := &fileIconSet{
+		byExt:    make(map[string]fileIcon),
+		fallback: fileIcon{Standard: raw.Default.Standard, Retro: raw.Default.Retro},
+	}
+	if err := validateFileIcon(staticDir, set.fallback); err != nil {
+		return nil, err
+	}
+	for _, t := range raw.Types {
+		icon := fileIcon{Standard: t.Standard, Retro: t.Retro}
+		if err := validateFileIcon(staticDir, icon); err != nil {
+			return nil, err
+		}
+		for _, ext := range t.Extensions {
+			set.byExt[strings.ToLower(strings.TrimPrefix(ext, "."))] = icon
+		}
+		if t.Mime == "" {
+			continue
+		}
+		if strings.HasSuffix(t.Mime, "/*") {
+			set.byMime = append(set.byMime, mimeRule{pattern: strings.TrimSuffix(t.Mime, "*"), prefix: true, icon: icon})
+		} else {
+			set.byMime = append(set.byMime, mimeRule{pattern: strings.ToLower(t.Mime), icon: icon})
+		}
+	}
+	return set, nil
+}
+
+func validateFileIcon(staticDir string, icon fileIcon) error {
+	if icon.Standard != "" {
+		if err := validateFileIconPath(staticDir, "standard", icon.Standard); err != nil {
+			return err
+		}
+	}
+	if icon.Retro != "" {
+		if err := validateFileIconPath(staticDir, "retro", icon.Retro); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func validateFileIconPath(staticDir, theme, name string) error {
+	if strings.Contains(name, "/") || strings.Contains(name, "\\") || strings.Contains(name, "..") {
+		return fmt.Errorf("invalid %s file icon path %q", theme, name)
+	}
+	path := filepath.Join(staticDir, "file-icons", theme, name)
+	info, err := os.Stat(path)
+	if err != nil {
+		return fmt.Errorf("missing %s file icon %q: %w", theme, name, err)
+	}
+	if info.IsDir() {
+		return fmt.Errorf("%s file icon %q is a directory", theme, name)
+	}
+	return nil
+}
+
+// lookup resolves a file's icon from its name (extension) first, falling back to
+// its content type, then to the default icon. Extension wins because stored
+// content types are often the generic application/octet-stream.
+func (s *fileIconSet) lookup(name, contentType string) fileIcon {
+	if s == nil {
+		return fileIcon{}
+	}
+	if ext := strings.ToLower(strings.TrimPrefix(filepath.Ext(name), ".")); ext != "" {
+		if icon, ok := s.byExt[ext]; ok {
+			return icon
+		}
+	}
+
+	ct := strings.ToLower(strings.TrimSpace(contentType))
+	if i := strings.IndexByte(ct, ';'); i >= 0 {
+		ct = strings.TrimSpace(ct[:i])
+	}
+	if ct != "" && ct != "application/octet-stream" {
+		for _, rule := range s.byMime { // exact matches first
+			if !rule.prefix && rule.pattern == ct {
+				return rule.icon
+			}
+		}
+		for _, rule := range s.byMime { // then major-type prefixes
+			if rule.prefix && strings.HasPrefix(ct, rule.pattern) {
+				return rule.icon
+			}
+		}
+	}
+	return s.fallback
+}
+
+// fileIconURL builds the /static URL for an icon filename in the given theme
+// directory ("standard" or "retro").
+func fileIconURL(theme, name string) string {
+	if name == "" {
+		return ""
+	}
+	return "/static/file-icons/" + theme + "/" + name
+}