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-browser/src/context.rs

@@ -11,6 +11,14 @@ pub struct BrowserContext {
     pub robots_cache: Arc<RobotsCache>,
     pub obey_robots: bool,
     pub stealth: bool,
+    /// When true, CDP-driven navigation to file:// URLs is permitted.
+    /// Default is false: a remote CDP client cannot point the browser
+    /// at /etc/shadow even if Obscura is running as a privileged user.
+    /// Flip on via `obscura serve --allow-file-access` for legitimate
+    /// local-HTML testing workflows. The CLI's own `obscura fetch
+    /// file://...` path is unaffected because it does not go through
+    /// the CDP server.
+    pub allow_file_access: bool,
 }
 
 impl BrowserContext {
@@ -26,6 +34,7 @@ impl BrowserContext {
             robots_cache: Arc::new(RobotsCache::new()),
             obey_robots: false,
             stealth: false,
+            allow_file_access: false,
         }
     }
 
@@ -66,6 +75,7 @@ impl BrowserContext {
             robots_cache: Arc::new(RobotsCache::new()),
             obey_robots: false,
             stealth,
+            allow_file_access: false,
         }
     }