Replace the 24px edge-swipe with a router-level interactive back gesture: a
rightward drag in the left ~38% band tracks the finger while a cheap themed
backdrop parallaxes in, and on release commits the back-navigation (past ~40%
of the width or a rightward flick) or snaps back. The live parent mounts only
on commit, which suppresses that one route slide for a seamless hand-off.
Pure decision logic (shouldArm/lockAxis/decideCommit + the per-gesture
controller) lives in lib/backswipe.ts with unit tests; the DOM wiring, hit-test
and settle animation in lib/backswipe.svelte.ts. The gesture yields to native
vertical scroll, skips the rack / draggable pending tiles / text inputs / the
board while zoomed-in, and is disabled inside Telegram (native BackButton owns
it). parentPath in App.svelte is now the single source for both the Telegram
button and the swipe.
Tests: backswipe unit (13) + e2e (3; Chromium+WebKit: commit, zoomed-in skip,
vertical scroll). Docs: UI_DESIGN.md Back bullet.
Ilia Denisov