feat: support file:// URL scheme for local HTML testing

Allow loading local HTML/CSS/JS files via the file:// protocol,
enabling Obscura to be used for testing static sites without
running a local HTTP server.

Changes:
- Allow "file" scheme in validate_url() and validate_fetch_url()
- Add fetch_file_url() to read local files and return synthetic
  HTTP responses with appropriate content-type headers
- Skip private IP / localhost checks for file:// URLs

Closes #16

crates/obscura-js/src/ops.rs

@@ -551,13 +551,17 @@ fn glob_match(pattern: &str, url: &str) -> bool {
 
 fn validate_fetch_url(url: &url::Url) -> Result<(), String> {
     let scheme = url.scheme();
-    if scheme != "http" && scheme != "https" {
+    if scheme != "http" && scheme != "https" && scheme != "file" {
         return Err(format!(
-            "Forbidden URL scheme '{}' - only http and https are allowed",
+            "Forbidden URL scheme '{}' - only http, https, and file are allowed",
             scheme
         ));
     }
 
+    if scheme == "file" {
+        return Ok(());
+    }
+
     if let Some(host) = url.host() {
         match host {
             url::Host::Ipv4(ip) => {