fix(hub): send a CSP and protective headers on every response

The SPA shell and its assets went out with no Content-Security-Policy
and no X-Content-Type-Options / Referrer-Policy / X-Frame-Options — so
an injection that reached the SPA (rendered third-party OpenGraph
data, a federated group name, chat content) had nothing stopping it
from loading more code or exfiltrating to any host, and the page
could be framed by any site.

A middleware in `create_app` now adds all four to every response.
`webapp.CSP` is deliberately the *same* policy the desktop client's
protocol handler already enforces on these exact UI files, plus the
two reCAPTCHA hosts the sign-up widget needs: `default-src 'none'`,
`script-src 'self' 'wasm-unsafe-eval' <recaptcha>` (the hub's own
origin is not a script source — T3), `style-src 'self'
'unsafe-inline'` (htm/preact inline `style=` only, nothing executes),
`connect-src 'self' https: wss:`, `frame-ancestors 'none'`,
`base-uri 'none'`, `form-action 'none'`.

The shell's dead `<script>window.__MB_ASSET_V = ...</script>` is
removed (nothing has ever read it) so `script-src` needs no inline
allowance.

Needs verification against the running SPA — a mis-tuned CSP shows as
a blank page — but it matches a policy already proven with these
files under Electron.

Second-review L5 / third-review M5.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pG75yGK3NthNfyjH74omG

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