Steam desktop authentication support (SteamSDK, Auth branch, audience config, username seeding) (#4683)

> **Maintainer note:** part of the private OpenFront Steam release
(Milestone 2, tracked in Linear), not a public GitHub issue. Per the
contributor policy this needs an approved/assigned issue or a maintainer
label to avoid auto-close — flagging for a maintainer to handle.

Resolves: _n/a — private Steam-release work (see note above)_

## Description:

Adds the client-side pieces for authenticating the OpenFront **Steam
desktop build** (an Electron shell) against the existing API, mirroring
the established CrazyGames integration. The native Steamworks code lives
in the desktop shell and is exposed to this client through a
`window.openfrontDesktop.steam` bridge; this PR is the renderer half.

- **`SteamSDK.ts`** (new) — thin wrapper over the desktop shell's Steam
bridge (`isOnSteam` / `getTicket` / `getUser`), mirroring
`CrazyGamesSDK`; narrows the loosely-typed `window.openfrontDesktop`
locally rather than re-declaring it.
- **`Auth.ts`** — a Steam branch at the top of `doRefreshJwt()` plus
`doSteamLogin()`, exchanging a Steam Web-API ticket at `POST
/auth/steam` for a session JWT, exactly paralleling `doCrazyGamesLogin`.
Re-exchanges on expiry (the refresh cookie is cross-site-blocked from
the `app://` origin); no ticket / not on Steam falls through to the
existing flow.
- **`Api.ts`** — `getAudience()` now sources from `BOOTSTRAP_CONFIG`
(`ClientEnv.jwtAudience()`) instead of `window.location`, so the desktop
app (running at `app://openfront`) resolves the real API base.
Behavior-preserving on web, where the injected audience already equals
the hostname-derived one.
- **`UsernameInput.ts`** — seeds the in-game display name from the Steam
persona on first launch and persists it (unlike CrazyGames). Writes only
the local display name; never the account username.
- **`SteamLinkSignpost.ts`** (new) — a one-time, dismissible notice
shown on first Steam launch that account linking is coming.

## Please complete the following:

- [ ] I have added screenshots for all UI updates — ⚠️ _the new
first-launch link signpost is a UI addition; it renders only inside the
Steam desktop shell. Screenshots to be added by the maintainer._
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file — signpost strings go through
`translateText()` with keys added to `resources/lang/en.json`
(`steam.link_signpost`, `common.got_it`).
- [x] I have added relevant tests to the test directory — unit tests for
`SteamSDK` (incl. bridge-throws degradation), the `Auth.ts` Steam
exchange + fall-through, the `Api.ts` audience change, username seeding,
and the signpost gating. Full suite green (1976) + `tsc --noEmit` clean.

## Please put your Discord username so you can be contacted if a bug or
regression is found:

_⚠️ maintainer to fill_

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01BWxUzYb2uqjcBFQuNSJhMy

resources/lang/en.json

@@ -370,6 +370,7 @@
     "duration_minute_short": "min",
     "duration_second_short": "s",
     "enabled": "Enabled",
+    "got_it": "Got it",
     "map_default": "Map default",
     "month_apr": "April",
     "month_aug": "August",
@@ -1406,6 +1407,9 @@
     "toggle_achievements": "Toggle achievements",
     "water_nukes": "Water nukes"
   },
+  "steam": {
+    "link_signpost": "Have an existing OpenFront account? Account linking is coming in a later update."
+  },
   "store": {
     "already_subscribed": "Already subscribed.",
     "change_tier_failed": "Couldn't update your subscription. Please try again.",