feat(ui): first-run onboarding coachmarks

A one-time coachmark overlay walks a new player through the lobby and their
first game board: a light dimmed layer draws one tail-pointed hint bubble at a
time, advancing on a tap anywhere and removing itself for good after the last
hint. Two independent series (lobby: settings/stats/new game; game:
header/pass-exchange/hints/shuffle/rack), gated by a per-device persisted flag
and marked done only after the last hint, so an interrupted run replays from
the start. A deep-link into Settings -> Friends still triggers the lobby series
on the first trip back to the lobby.

Targets carry a data-coach attribute, so one positioning engine anchors the
bubble in both portrait and landscape, re-measuring each frame until the
geometry settles (route slide, hidden-banner reflow, fonts). The promo banner
hides while the overlay is up (app.coachActive); a hidden DebugPanel "Reset
visited" control replays the walk-through. Off by default in the mock build so
the Playwright smoke is unaffected; ?coach forces it on for the dedicated e2e.

Pure geometry (step lists, nextVisibleStep, placeBubble) in lib/coachmark.ts
(unit-tested); Coachmark.svelte renders. Docs: FUNCTIONAL(+ru) onboarding
story, UI_DESIGN coachmark section.

ui/src/App.svelte

@@ -8,6 +8,7 @@
   import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
   import WelcomeRedeemModal from './components/WelcomeRedeemModal.svelte';
+  import Coachmark from './components/Coachmark.svelte';
   import DebugPanel from './components/DebugPanel.svelte';
   import Login from './screens/Login.svelte';
   import Lobby from './screens/Lobby.svelte';
@@ -120,6 +121,7 @@
 <Toast />
 <StaleInviteModal />
 <WelcomeRedeemModal />
+<Coachmark />
 
 {#if routeIsLobby && !app.splashDone && !app.blocked && !app.bootError && !app.launchError}
   <Splash />