Embedded the frontend in the binary, and built it in the build script

`npm run sync` in web/ copies the build output (web/dist, from npm run build)
to internal/frontend/dist, which the binary embeds. Go can't embed from
outside the package's directory (web/ is also outside the Go module), hence
the copy. Only dist/.gitkeep is committed, so a fresh clone still compiles,
and a binary built without syncing first serves no frontend.

job-search serve serves server.frontend.dir if it is set, or else the
embedded build, or warns that there is no frontend. scripts/build.sh builds
and syncs the frontend before building, running npm ci first for a release
or when web/node_modules is missing.

web/CODE_CONVENTIONS.md

@@ -15,7 +15,8 @@ npm run lint           # eslint .
 npm run format:check   # prettier --check .
 npm run format         # prettier --write .: formats every file
 npm run generate:api   # regenerate src/types/api.ts from the backend's OpenAPI description
-npm run build          # typecheck, then vite build
+npm run build          # typecheck, then vite build into dist/
+npm run sync           # copy dist/ to ../internal/frontend/dist/, which the backend embeds
 npm run dev            # dev server; /api and /ws go to the backend (API_PROXY_TARGET, default localhost:9393)
 ```
 
@@ -79,7 +80,7 @@ if (isRelease(parsed)) { ... }
 | `src/primitives/` | Solid primitives: state and loading, no markup (`createX.ts`)                                      |
 | `src/components/` | Components, one exported component per file                                                        |
 | `src/lib/`        | Plain functions: no Solid, no DOM where avoidable                                                  |
-| `scripts/`        | Development scripts run by npm, in TypeScript (`generateApiTypes.ts`)                              |
+| `scripts/`        | Development scripts run by npm, in TypeScript (`generateApiTypes.ts`, `syncDist.ts`)               |
 
 - Components' files are PascalCase, named after the component
   (`MarkMenu.tsx`). Other files are usually camelCase (`format.ts`); the