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>

public/i18n/locale-en.json

@@ -10,6 +10,7 @@
     "user_not_found": "The requested user could not be found.",
     "user_banned": "Your account has been banned. Contact the admin for more information.",
     "repo_access_limited": "GitHub blocked access because the repository's organization restricts third-party OAuth apps. Ask an org owner to approve Anonymous GitHub under Settings → Third-party Access → OAuth app policy, or anonymize a personal fork instead.",
+    "repo_saml_enforcement": "The repository's organization enforces SAML single sign-on. Authorize your token for that organization (GitHub → Settings → Applications, or re-run the org's SSO sign-in), then retry. Alternatively, anonymize a personal fork.",
     "repo_not_found": "The repository was not found on GitHub. Check the URL and spelling, make sure you are signed in to the account that can see it, and confirm the repo isn't hidden under an org that restricts third-party app access.",
     "repo_empty": "The selected branch has no commits on GitHub. Push at least one commit, or pick a different branch, then retry.",
     "repo_not_accessible": "Anonymous GitHub cannot access this repository. Verify the repository exists and that Anonymous GitHub has been authorized for the owning organization.",
@@ -52,6 +53,7 @@
     "path_not_specified": "A file path must be specified.",
     "path_not_defined": "The file path has not been resolved yet.",
     "invalid_file_path": "The requested file path is not valid.",
+    "invalid_request": "The request is missing required fields or is malformed.",
     "no_file_selected": "Please select a file.",
     "file_not_found": "The requested file is not found.",
     "file_not_accessible": "The requested file is not accessible.",