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/components/Header.svelte

@@ -48,8 +48,11 @@
   </div>
   <!-- The ad banner lives inside the nav, directly under the title bar, so it sits in the
        same place on every screen — and in the game (grown nav) the spare height falls below
-       it (banner under the title, board pinned to the bottom). -->
-  {#if app.profile?.banner && app.profile.banner.campaigns.length}
+       it (banner under the title, board pinned to the bottom). It is hidden while a first-run
+       coachmark overlay is up (app.coachActive) so the scrolling strip does not run behind the
+       dimmed onboarding layer; the engine keeps rotating (module scope) and the strip reappears,
+       per this same condition, once onboarding closes. -->
+  {#if app.profile?.banner && app.profile.banner.campaigns.length && !app.coachActive}
     <AdBanner
       campaigns={app.profile.banner.campaigns}
       timings={app.profile.banner.timings}