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.

docs/FUNCTIONAL.md

@@ -25,6 +25,20 @@ theme, and links to the matching per-language Telegram channel; the game itself
 `/app/` (web), `/telegram/` (the Telegram Mini App) and `/vk/` (the VK Mini App). The landing's theme is ephemeral
 (it follows the system scheme, not the saved preference); its language choice is saved.
 
+### First-run onboarding
+The first time a player opens the app it walks them through the interface with a light
+**coachmark overlay**: a dimmed layer draws one hint bubble at a time, each pointing — with a
+tail — at a real control, and a **tap anywhere** advances to the next; after the last hint the
+overlay is gone for good. Two independent series run. The **lobby** series points at the
+⚙️ settings, ✏️ statistics and 🎲 new-game tabs. The **game** series, on the player's first game
+board, points at the scoreboard header (move history / chat / word-check), the 🔄 pass-and-exchange,
+🛟 hints and 🔀 shuffle controls, and the rack. A series counts as seen only **after its last hint**,
+so leaving mid-way replays it from the start next time; it is remembered per device. Arriving at the
+lobby is the lobby trigger, so a player who deep-links straight into Settings → Friends still gets the
+lobby walk-through on their first trip back. Both series work the same in portrait and landscape (the
+bubbles re-anchor to wherever the controls move) and the hint texts are localized (en/ru). The
+advertising banner hides while the overlay is up and returns when it closes.
+
 ### Identity & sessions
 A player arrives from a platform (Telegram first), via email login, or as an
 ephemeral guest. The gateway validates the credential once and mints a thin