fix(mcp): vertical_scrape uses Firefox profile, not default Chrome

Reddit's .json API rejects the wreq-Chrome TLS fingerprint with a
403 even from residential IPs. Their block list includes known
browser-emulation library fingerprints. wreq-Firefox passes. The
CLI `vertical` subcommand already forced Firefox; MCP
`vertical_scrape` was still falling back to the long-lived
`self.fetch_client` which defaults to Chrome, so reddit failed
on MCP and nobody noticed because the earlier test runs all had
an API key set that masked the issue.

Switched vertical_scrape to reuse `self.firefox_or_build()` which
gives us the cached Firefox client (same pattern the scrape tool
uses when the caller requests `browser: firefox`). Firefox is
strictly-safer-than-Chrome for every vertical in the catalog, so
making it the hard default for `vertical_scrape` is the right call.

Verified end-to-end from a clean shell with no WEBCLAW_API_KEY:
- MCP reddit: 679ms, post/author/6 comments correct
- MCP instagram_profile: 1157ms, 18471 followers

No change to the `scrape` tool -- it keeps the user-selectable
browser param.

Bumps version to 0.5.3.

crates/webclaw-mcp/src/server.rs

@@ -749,16 +749,21 @@ impl WebclawMcp {
         Parameters(params): Parameters<VerticalParams>,
     ) -> Result<String, String> {
         validate_url(&params.url)?;
-        // Reuse the long-lived default FetchClient. Extractors accept
-        // `&dyn Fetcher`; FetchClient implements the trait so this just
-        // works (see webclaw_fetch::Fetcher and client::FetchClient).
-        let data = webclaw_fetch::extractors::dispatch_by_name(
-            self.fetch_client.as_ref(),
-            &params.name,
-            &params.url,
-        )
-        .await
-        .map_err(|e| e.to_string())?;
+        // Use the cached Firefox client, not the default Chrome one.
+        // Reddit's `.json` endpoint rejects the wreq-Chrome TLS
+        // fingerprint with a 403 even from residential IPs (they
+        // ship a fingerprint blocklist that includes common
+        // browser-emulation libraries). The wreq-Firefox fingerprint
+        // still passes, and Firefox is equally fine for every other
+        // vertical in the catalog, so it's a strictly-safer default
+        // for `vertical_scrape` than the generic `scrape` tool's
+        // Chrome default. Matches the CLI `webclaw vertical`
+        // subcommand which already uses Firefox.
+        let client = self.firefox_or_build()?;
+        let data =
+            webclaw_fetch::extractors::dispatch_by_name(client.as_ref(), &params.name, &params.url)
+                .await
+                .map_err(|e| e.to_string())?;
         serde_json::to_string_pretty(&data)
             .map_err(|e| format!("failed to serialise extractor output: {e}"))
     }