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/lib/i18n/en.ts

@@ -65,6 +65,17 @@ export const en = {
   'new.searchHint':
     'Finding an opponent can sometimes take a while. If you do not want to wait, close the app after starting the game and come back in a couple of minutes.',
 
+  // First-run coachmark hints (components/Coachmark.svelte). The leading emoji in each comment
+  // names the UI element the bubble's tail points at.
+  'onboarding.lobbySettings': 'Friends, game & profile settings, feedback', // ⚙️
+  'onboarding.lobbyStats': 'Stats refresh after each game with an opponent', // ✏️
+  'onboarding.lobbyNew': 'Practise against the AI', // 🎲
+  'onboarding.gameHeader': 'Move history, chat and word check', // scoreboard strip
+  'onboarding.gameTurn': 'Pass a turn, swap tiles', // 🔄
+  'onboarding.gameHints': 'Available hints', // 🛟
+  'onboarding.gameShuffle': 'Shuffle your tiles', // 🔀
+  'onboarding.gameRack': 'Pick a tile and place it on the board', // rack
+
   'game.bag': '{n} in the bag',
   'game.bagEmpty': 'Bag is empty',
   'game.hints': 'Hints {n}',