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/config.ts

@@ -1,4 +1,5 @@
 import { resolve } from "path";
+import { randomBytes } from "crypto";
 
 interface Config {
   SESSION_SECRET: string;
@@ -37,7 +38,10 @@ interface Config {
   RATE_LIMIT: number;
 }
 const config: Config = {
-  SESSION_SECRET: "SESSION_SECRET",
+  // Predictable defaults are dangerous: a known SESSION_SECRET lets anyone
+  // forge session cookies. Default to empty and resolve below — random in
+  // dev, required in production. See the post-env block.
+  SESSION_SECRET: "",
   CLIENT_ID: "CLIENT_ID",
   CLIENT_SECRET: "CLIENT_SECRET",
   GITHUB_TOKEN: "",
@@ -99,4 +103,42 @@ for (const conf in process.env) {
   }
 }
 
+// Harden security-sensitive secrets that still hold an unset/predictable
+// value after reading the environment (CWE-798).
+const isProduction = process.env.NODE_ENV === "production";
+
+// SESSION_SECRET: a known value allows session forgery. Require it in
+// production; in development fall back to a per-process random value so the
+// app still boots without shipping a guessable secret.
+if (!config.SESSION_SECRET || config.SESSION_SECRET === "SESSION_SECRET") {
+  if (isProduction) {
+    throw new Error(
+      "SESSION_SECRET must be set to a strong random value in production"
+    );
+  }
+  config.SESSION_SECRET = randomBytes(32).toString("hex");
+  // eslint-disable-next-line no-console
+  console.warn(
+    "SESSION_SECRET not set — generated a random development secret. " +
+      "Sessions will not persist across restarts. Set SESSION_SECRET in production."
+  );
+}
+
+// Refuse to start in production with the placeholder OAuth credentials or the
+// default database password baked into the image.
+if (isProduction) {
+  const insecureDefaults: [string, string][] = [
+    ["CLIENT_ID", "CLIENT_ID"],
+    ["CLIENT_SECRET", "CLIENT_SECRET"],
+    ["DB_PASSWORD", "password"],
+  ];
+  for (const [key, badValue] of insecureDefaults) {
+    if ((config as unknown as Record<string, unknown>)[key] === badValue) {
+      throw new Error(
+        `${key} is using its insecure default value; set it via the environment in production`
+      );
+    }
+  }
+}
+
 export default config;