feat(app): preview HTML files in the file pane (#2712)

* feat(app): preview HTML files in the file pane

Renders .html/.htm files as a page in the file pane, with the same
Preview/Source toggle Markdown already has. The main use is reading
richer visual plans an agent wrote as a self-contained HTML page.

A preview is a viewer, not a browser. Each document is prefixed with a
doctype and a strict policy, so the policy is always the first element
and always lands in the parser-created head: inline script and style
run, and remote subresources, fetch, XHR, WebSocket, beacon, and form
posts are refused. The frame has an opaque origin, so it cannot reach
the app's DOM and storage and cookie APIs throw inside it.

One gap is left on web and cannot be closed there: a sandboxed
document may navigate itself, and no CSP directive in current browsers
prevents it -- navigate-to was dropped from CSP3 and is unenforced, and
meta refresh needs no script (both verified against the Chromium the
app ships against). The opaque origin bounds it to the page's own
contents, and SECURITY.md documents it. Native narrows it further by
refusing navigation after the initial document, with the caveat that
the decision runs in app JS.

Self-contained pages only -- a page that pulls a CDN script renders
unstyled, and Source still shows everything.

* fix(app): harden HTML preview behavior and tests

---------

Co-authored-by: Nicholas Salgueiro <nicholas.salgueiro@britecore.com>
Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

SECURITY.md

@@ -66,6 +66,18 @@ CORS is not a complete security boundary. It controls which browser origins can
 
 Paseo validates the `Host` header on every HTTP request and every WebSocket upgrade against an allowlist (Vite-style semantics). By default, only `localhost`, `*.localhost`, and any literal IP address (IPv4 or IPv6) are accepted. Additional hostnames can be configured via `hostnames` in `config.json` or the `PASEO_HOSTNAMES` env var (comma-separated; entries beginning with `.` match a domain and its subdomains; the value `true` disables the allowlist entirely). Requests with unrecognized hosts are rejected with `403 Host not allowed`.
 
+## HTML file preview
+
+Previewing an `.html` file in the file pane renders it as a page, so markup an agent wrote — or markup that arrived with a repo you cloned — executes when you open it. The preview is built to contain that, not to trust it.
+
+The document loads with an opaque origin and a policy that permits inline script and style and refuses everything else: no remote script, font, image, or media; no `fetch`, XHR, WebSocket, or beacon; no form posts; no plugins; no nested frames. It has no access to Paseo's DOM, and storage and cookie APIs throw inside it rather than returning anything. It cannot navigate the top window, and it cannot open popups. It cannot read any file but itself.
+
+One gap remains on web and desktop: a sandboxed document may navigate _itself_, and no CSP directive in current browsers prevents that. `navigate-to` was dropped from CSP Level 3 and is not enforced, and `<meta http-equiv="refresh">` needs no script at all. A hostile page can therefore reach a server by navigating away, carrying data available inside the preview, such as its own contents, browser and device properties, user input inside the page, and your IP address. It cannot read Paseo, another file, storage, or cookies.
+
+Native builds narrow this gap rather than closing it outright. The WebView refuses every navigation after the initial document, but that decision is made in the app's JavaScript, and on Android the WebView falls back to allowing a navigation when the decision doesn't come back in time. Treat it as a strong mitigation, not a guarantee: if the JS thread is stalled at the moment a page navigates, the same leak is possible there too.
+
+If you don't trust a page, read it in `Source`, which executes nothing. Source is available as an editable view on supported web hosts and a read-only view everywhere else.
+
 ## Agent authentication
 
 Paseo wraps agent CLIs (Claude Code, Codex, OpenCode) but does not manage their authentication. Each agent provider handles its own credentials. Paseo never stores or transmits provider API keys. Agents run in your user context with your existing credentials.