Security hardening + gist UI fixes (#731)

* security: harden against XSS, ReDoS, path traversal, and injection

Defensive fixes across the server, storage, and viewer:

- XSS (CWE-79): sanitise rendered notebooks with DOMPurify, escape file
  names interpolated into AngularJS expressions (escapeNgString), set
  Mermaid securityLevel to 'strict', and stop urlRel2abs from returning
  javascript:/vbscript:/data:text/html URLs.
- Path traversal / zip-slip (CWE-22/23/24): validate URL-derived path
  components before they reach the storage layer (file/webview routes +
  StorageBase.assertSafePath) and sanitise zip entry names on extract for
  both the filesystem and S3 backends.
- ReDoS (CWE-1333): escape anonymization terms with catastrophic
  backtracking shapes to literals instead of compiling them as regexes.
- Secret hardening (CWE-798): require SESSION_SECRET / OAuth creds / DB
  password in production, random dev SESSION_SECRET fallback.
- Rate-limit spoofing (CWE-290): derive request.ip via trust-proxy hop
  count instead of the client-settable cf-connecting-ip header.
- NoSQL injection (CWE-943): allow only plain field paths as admin sort keys.
- Reject malformed streamer requests missing required string fields.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(ui): make gists reachable/visible and clarify the ZIP button

- Gist & PR routes now accept a trailing slash (/gist/:id/:path*?), so the
  dashboard links (which end in "/") resolve to the gist/PR page instead of
  falling through to the 404 route (#725).
- Gist viewer picks the default tab after content loads, defaulting to
  "files" when files exist; previously the ng-init ran before the async
  load and a files-only gist rendered blank under the hidden comments tab.
- Explorer toolbar: relabel ZIP to "Full repo ZIP" with a tooltip, and add
  tooltips to Raw/Download clarifying they apply to the current file (#721).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: report SAML-enforced orgs clearly instead of "token expired"

When a repo's organization enforces SAML SSO, GitHub returns a 403 whose
message differs from the OAuth-App-restriction case. That 403 fell through
to the generic handler and surfaced as "token_expired", pushing users to
re-login when the real fix is authorizing their token for the org. Detect
the "SAML enforcement" message and raise a dedicated, actionable error
instead (#379, #550).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* security: catch nested quantified groups in ReDoS guard and backslash path traversal

- hasCatastrophicBacktracking now scans across nested parens ([\s\S]*?)
  so shapes like ((a+))+ are detected; comment reframed as a heuristic
  backstop rather than a proof.
- file route path-traversal check now rejects backslash separators and a
  leading backslash, covering Windows-style "..\" payloads (CWE-22/25).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(dev): track dev-proxy script, ignore .DS_Store and .claude/

scripts/dev-proxy.js is referenced by the "dev:ui" npm script but was
never committed, breaking the command on a fresh clone. Add it and
ignore local-only macOS/Claude Code files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/server/index.ts

@@ -109,6 +109,10 @@ export default async function start() {
     })
   );
   app.set("etag", "strong");
+  // Trust exactly TRUST_PROXY proxy hops so Express derives request.ip from
+  // the right X-Forwarded-For entry. This is what makes request.ip
+  // trustworthy for rate limiting instead of a client-spoofable header.
+  app.set("trust proxy", config.TRUST_PROXY);
 
   // handle session and connection
   app.use(initSession());
@@ -134,9 +138,11 @@ export default async function start() {
     request: express.Request,
     _response: express.Response
   ): string {
-    if (request.headers["cf-connecting-ip"]) {
-      return request.headers["cf-connecting-ip"] as string;
-    }
+    // Use request.ip, which Express resolves from X-Forwarded-For honouring
+    // the configured "trust proxy" hop count. Do NOT key off the
+    // cf-connecting-ip header directly: when the server isn't actually behind
+    // Cloudflare a client can set that header to an arbitrary value per
+    // request and trivially bypass the rate limiter (CWE-290).
     if (!request.ip && request.socket.remoteAddress) {
       logger.warn("request.ip is missing");
       return request.socket.remoteAddress;