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.

Cargo.toml

@@ -3,7 +3,7 @@ resolver = "2"
 members = ["crates/*"]
 
 [workspace.package]
-version = "0.5.2"
+version = "0.5.3"
 edition = "2024"
 license = "AGPL-3.0"
 repository = "https://github.com/0xMassi/webclaw"