fix(cdp): gate file:// navigation behind --allow-file-access

GHSA-q55h-vfv9-qcr5. Any process able to reach the CDP port could
send Page.navigate {"url":"file:///etc/shadow"} and read arbitrary
local files (the process's UID's worth of read access). When obscura
runs as root inside Docker or as a systemd service that translates
straight into /etc/shadow, /root/.ssh/id_rsa, /proc/1/environ, etc.

Validate at the CDP boundary, not in obscura-net, so direct CLI
callers ("obscura fetch file://...") keep working. A new
BrowserContext.allow_file_access flag defaults to false; Page.navigate
returns an error when the requested URL is file:// and the flag is
off. CdpContext::new_with_security threads the flag from the server
entry point, and "obscura serve --allow-file-access" opts in for
local-HTML testing workflows.

Tested:
- Default obscura serve: CDP Page.navigate {file:///etc/passwd} ->
  refused with descriptive error.
- obscura serve --allow-file-access: same call succeeds.
- obscura fetch file://... CLI path: unchanged, still reads the file.
- All earlier PoCs (GHSA-6pj9 JS->file SOP, GHSA-4789 sub-resource
  SSRF, GHSA-4789 CSS template injection) still pass.
- cargo test --workspace clean.

crates/obscura-cdp/src/dispatch.rs

@@ -57,12 +57,23 @@ impl CdpContext {
         stealth: bool,
         user_agent: Option<String>,
     ) -> Self {
-        let default_context = Arc::new(BrowserContext::with_full_options(
+        Self::new_with_security(proxy, stealth, user_agent, false)
+    }
+
+    pub fn new_with_security(
+        proxy: Option<String>,
+        stealth: bool,
+        user_agent: Option<String>,
+        allow_file_access: bool,
+    ) -> Self {
+        let mut ctx = BrowserContext::with_full_options(
             "default".to_string(),
             proxy,
             stealth,
             user_agent,
-        ));
+        );
+        ctx.allow_file_access = allow_file_access;
+        let default_context = Arc::new(ctx);
         // Pre-seed with the default-frame execution context ids that
         // `Runtime.enable` (1) and post-navigation re-emission (2) advertise
         // via Runtime.executionContextCreated. Anything else has to be