chore(deps): bump mermaid to 11.16.1 (#52)

## Related Issue

No issue. The problem is described below.

## Problem

Dependabot reports nine open alerts against `main`. Five of them are
`mermaid@11.16.0`, a direct dependency of the documentation site:

| Severity | Advisory |
| --- | --- |
| Medium | Configuration APIs allow prototype pollution |
| Medium | Architecture diagrams are vulnerable to prototype pollution |
| Medium | CSS injection applying to sibling elements of the diagram |
| Medium | Radar diagrams are vulnerable to denial of service |
| Low | XY charts are vulnerable to an infinite loop denial of service |

All five are fixed in `11.16.1`, which the declared `^11.16.0` range
already permits — only the lockfile held the old version.

## What changed

- `mermaid` moves to `11.16.1` in `docs/package.json` and the lockfile.
- The `pnpmDeps` hash in `flake.nix` is refreshed, because any lockfile
change invalidates it.

The remaining four alerts are out of scope and cannot be fixed here.
They are `vite@5.4.21` (one high, two medium) and its `esbuild@0.21.5`
dependency, both reached only through `vitepress@1.6.4`, which is the
latest stable release and pins `vite: ^5.4.14`. The fix exists only in
`vite@6.4.3` with no 5.x backport, so clearing them needs VitePress 2.0,
currently an alpha. Every one of those advisories is a
development-server issue, and the vulnerable copy exists only under
`docs/` — the other five workspaces already pin `vite ^6.4.3`.

## Verification

- `nix build .#pythinker-code.pnpmDeps` passes with the new hash. The
old hash was confirmed stale by forcing a mismatch: a fixed-output
derivation is content-addressed, so a local build reuses the cached path
and reports success even when the lockfile has moved.
- `node scripts/check-nix-workspace.mjs` passes.
- `pnpm exec vitest run` passes: 651 files, 9949 tests.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/Pythoughts-labs/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue, or explained the problem above.
- [x] I have added tests that prove my feature works. — Not applicable;
this is a dependency bump with no behaviour change.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset. — No
changeset: the bump touches only the documentation site's dependencies
and does not enter the CLI bundle.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.

Author: elkaix

docs/package.json

@@ -12,7 +12,7 @@
     "vitepress": "^1.6.4"
   },
   "dependencies": {
-    "mermaid": "^11.16.0",
+    "mermaid": "^11.16.1",
     "vitepress-plugin-llms": "^1.13.4",
     "vitepress-plugin-mermaid": "^2.0.17"
   }

flake.nix

@@ -148,7 +148,7 @@
               inherit (finalAttrs) pname version src pnpmWorkspaces;
               inherit pnpm;
               fetcherVersion = 3;
-              hash = "sha256-GKfwTPY1YSCXyTICa/T2SrkqitM1EL/7l3Ry0VfNtY0=";
+              hash = "sha256-iNpwl+jjkjkDHUs4BdU1VxUjz4rgYDDPNzJQWmjTkz0=";
             };
 
             nativeBuildInputs = [