feat(staticweb): add path prefix stripping to all filesystem providers

Add PrefixStrippingProvider interface and implement it in all providers
(EmbedFSProvider, LocalFSProvider, ZipFSProvider) to support serving
files from subdirectories at the root level.

pkg/server/staticweb/providers/local.go

@@ -4,13 +4,18 @@ import (
 	"fmt"
 	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
+	"strings"
+	"sync"
 )
 
 // LocalFSProvider serves files from a local directory.
 type LocalFSProvider struct {
-	path string
-	fs   fs.FS
+	path        string
+	stripPrefix string
+	fs          fs.FS
+	mu          sync.RWMutex
 }
 
 // NewLocalFSProvider creates a new LocalFSProvider for the given directory path.
@@ -39,8 +44,28 @@ func NewLocalFSProvider(path string) (*LocalFSProvider, error) {
 }
 
 // Open opens the named file from the local directory.
+// If a strip prefix is configured, it prepends the prefix to the requested path.
+// For example, with stripPrefix="/dist", requesting "/assets/style.css" will
+// open "/dist/assets/style.css" from the local filesystem.
 func (p *LocalFSProvider) Open(name string) (fs.File, error) {
-	return p.fs.Open(name)
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+
+	// Apply prefix stripping by prepending the prefix to the requested path
+	actualPath := name
+	if p.stripPrefix != "" {
+		// Clean the paths to handle leading/trailing slashes
+		prefix := strings.Trim(p.stripPrefix, "/")
+		cleanName := strings.TrimPrefix(name, "/")
+
+		if prefix != "" {
+			actualPath = path.Join(prefix, cleanName)
+		} else {
+			actualPath = cleanName
+		}
+	}
+
+	return p.fs.Open(actualPath)
 }
 
 // Close releases any resources held by the provider.
@@ -63,6 +88,9 @@ func (p *LocalFSProvider) Path() string {
 // For local directories, os.DirFS automatically picks up changes,
 // so this recreates the DirFS to ensure a fresh view.
 func (p *LocalFSProvider) Reload() error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
 	// Verify the directory still exists
 	info, err := os.Stat(p.path)
 	if err != nil {
@@ -78,3 +106,19 @@ func (p *LocalFSProvider) Reload() error {
 
 	return nil
 }
+
+// WithStripPrefix sets the prefix to strip from requested paths.
+// For example, WithStripPrefix("/dist") will make files at "/dist/assets"
+// accessible via "/assets".
+func (p *LocalFSProvider) WithStripPrefix(prefix string) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.stripPrefix = prefix
+}
+
+// StripPrefix returns the configured strip prefix.
+func (p *LocalFSProvider) StripPrefix() string {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+	return p.stripPrefix
+}