feat(build): add a standalone mode for deploys with no backend

[mcdope]

npx vite build --mode standalone produces a build with multiplayer switched off, for any deploy served as plain static files that cannot reach the signaling server.

The problem

The gate already existed:

// main.ts
const MULTIPLAYER_SERVER_CONFIGURED = Boolean(import.meta.env.VITE_MULTIPLAYER_SERVER_URL);

with updateMultiplayerTabEnabled hiding the tab when it is unset. What was missing was a way to ask for it — a default vite build runs in production mode and loads .env.production.local, so the URL is always present and the tab always appears.

On a host the signaling server does not accept, that advertises a mode which cannot connect. Found by uploading to itch.io and clicking it.

Named for what it is

Nothing here is itch-specific — it applies to any subpath host, any fork without a server of its own, any static mirror. An earlier draft called the mode itch, which was simply the wrong noun.

The honest limitation

.env.standalone sets the variable to empty explicitly, rather than letting the mode work by having no env file at all, so the intent is greppable in one place.

That file is documentation, not a guarantee — and this was measured rather than assumed. Vite's precedence is:

.env  <  .env.local  <  .env.[mode]  <  .env.[mode].local

Dropping a .env.standalone.local in place brings the Multiplayer tab straight back. I tested it, and it behaves identically whether the tracked file exists or not — so the explicit file buys discoverability, not safety. My first instinct was that explicit-empty would be more robust than absence; it isn't, and the test is what said so.

The only check that holds is on the built artifact:

[...document.querySelectorAll("#launch-tabs .tab-btn")]
  .filter(b => b.offsetParent !== null).map(b => b.textContent.trim())

Verification — both directions

The risk here is symmetric: the mode has to work, and it must not leak into the real deploy.

Build	Tabs	Multiplayer
--mode standalone	Local / Repo / Demos / ⚙	hidden ✅
default vite build	Local / Repo / Demos / Multiplayer / ⚙	still present ✅

Plus the usual gate: npm run build clean with bundle hygiene, vitest --dir src 3,682 passing across 135 files, docPins 72 passing.

Not included

No README entry yet — the mode is documented in .env.standalone's own comment and is greppable from there. Say if you would rather it were listed under Development Scripts too.

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https://claude.ai/code/session_017ncJfux8GDacSTeLDcqrhr