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/interfaces.go

@@ -34,6 +34,29 @@ type ReloadableProvider interface {
 	Reload() error
 }
 
+// PrefixStrippingProvider is an optional interface that providers can implement
+// to support stripping path prefixes from requested paths.
+// This is useful when files are stored in a subdirectory but should be accessible
+// at the root level (e.g., files at "/dist/assets" accessible via "/assets").
+type PrefixStrippingProvider interface {
+	// WithStripPrefix sets the prefix to strip from requested paths.
+	// For example, WithStripPrefix("/dist") will make files at "/dist/assets"
+	// accessible via "/assets".
+	WithStripPrefix(prefix string)
+
+	// StripPrefix returns the configured strip prefix.
+	StripPrefix() string
+}
+
+// WithStripPrefix is a helper function that sets the strip prefix on a provider
+// if it implements PrefixStrippingProvider. Returns the provider for method chaining.
+func WithStripPrefix(provider FileSystemProvider, prefix string) FileSystemProvider {
+	if p, ok := provider.(PrefixStrippingProvider); ok {
+		p.WithStripPrefix(prefix)
+	}
+	return provider
+}
+
 // CachePolicy defines how files should be cached by browsers and proxies.
 // Implementations must be safe for concurrent use.
 type CachePolicy interface {