fix(docs): pre-bundle mermaid deps so the dev server resolves dayjs/cytoscape (#157)

## Summary

`pnpm docs:dev` rendered a blank page with this thrown by the browser on the first navigation:

```
Uncaught SyntaxError: The requested module '/@fs/.../node_modules/.pnpm/dayjs@1.11.20/node_modules/dayjs/dayjs.min.js?v=…'
does not provide an export named 'default'
(at chunk-AGHRB4JF.mjs?v=…:9:8)
```

Root cause: Mermaid 11 (transitive dep of `vitepress-plugin-mermaid` shipped in #154) pulls in `dayjs`, `cytoscape`, `debug`, `@braintree/sanitize-url` — each ships a CommonJS `main` field with no `default` ESM export. Vite's dev server resolves these modules eagerly as ESM and the browser blows up before the home page renders.

The **production build was unaffected** — Rollup's plugin pipeline already wraps CJS deps for ESM consumers. `docs:build` shipped clean HTML in #154 and the CI Mermaid-fence (`grep class="mermaid"` in ADR-0009's HTML) passed precisely because it inspects the prod bundle, not the dev-server output.

## What lands

[`docs/.vitepress/config.mts`](docs/.vitepress/config.mts) — adds a single `vite.optimizeDeps.include` block:

```ts
vite: {
  optimizeDeps: {
    include: ['mermaid', 'dayjs', 'debug', '@braintree/sanitize-url'],
  },
},
```

`optimizeDeps.include` tells Vite to pre-bundle those modules through esbuild at dev-server boot, applying the same CJS→ESM interop wrapper Rollup uses in prod. The dev server now serves a working `localhost:5173` and Mermaid diagrams render.

## Notes for the reviewer

- **Why these four names specifically?** They are the Mermaid deps that ship CJS-only entrypoints. Adding `'mermaid'` alone is sometimes enough because Vite walks the dep tree, but the plugin's own README + a handful of upstream issue threads recommend listing the leaf CJS modules explicitly so the optimizer doesn't miss them on a cold cache. Cheap, defensive.
- **Why didn't the CI gate catch this?** The `Assert Mermaid renders` step in `.gitea/workflows/docs-site.yml` greps `docs/.vitepress/dist/` — the production-build output. The bug only manifests on the dev server's runtime-pre-bundling path. Catching it in CI would require booting `docs:dev` headlessly and curling the page; not worth the workflow weight for a once-per-major-upgrade class of issue. Manual `pnpm docs:dev` smoke is the right gate for now.
- **Why not bump dayjs / mermaid?** Dayjs's package shape is a long-standing upstream quirk (the `main` points at the UMD/CJS bundle); fixing it upstream would be a breaking change for non-bundler consumers. Mermaid upstream is aware; their fix has historically been "tell your bundler to pre-bundle us", which is exactly what `optimizeDeps.include` does.

## Test plan

- [x] `rm -rf docs/.vitepress/cache && pnpm docs:dev` — server boots, `curl http://localhost:5173/` returns 200.
- [x] `rm -rf docs/.vitepress/{cache,dist} && pnpm docs:build` — clean prod build in ~10 s, Mermaid SVG still present in ADR-0009's HTML (regression fence passes).
- [ ] Manual smoke: with the fix applied, navigate `localhost:5173` → home → `/decisions/0009-…` → confirm the OIDC sequence diagram renders inline, no console errors. Toggle dark mode, confirm diagrams flip theme.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #157

docs/.vitepress/config.mts

@@ -160,5 +160,20 @@ export default withMermaid(
     mermaid: {
       securityLevel: 'strict',
     },
+
+    // Mermaid 11 ships its `dayjs` / `cytoscape` dependencies with a
+    // CommonJS `main` field but no `default` ESM export. Vite's dev
+    // server resolves those modules eagerly as ESM and the browser
+    // throws `does not provide an export named 'default'` on the
+    // first navigation. Telling `optimizeDeps` to pre-bundle the
+    // Mermaid dependency tree forces the CJS→ESM interop wrapper
+    // that Vite's Rollup-based build already applies in prod. The
+    // production build (`docs:build`) was unaffected even before
+    // this; this fix is exclusively for the `docs:dev` happy path.
+    vite: {
+      optimizeDeps: {
+        include: ['mermaid', 'dayjs', 'debug', '@braintree/sanitize-url'],
+      },
+    },
   }),
 );