feat(pwa): linear-style create/edit on mobile — chip bar, sheet pickers, drafts, back guard (WI-1327)

Follow-ups from the WI-1327 mobile editing rework:

- The create page becomes a composition surface: borderless hero
  title/description plus a footer chip bar for the configured system
  properties. Priority and assignee edit through single-select sheets;
  milestone and labels use a new multi-select MobileOptionSheet mode
  with a Done button; dates, points, and estimate are inline chip
  inputs. The desktop pickers no longer render on /m.
- Editor pages register a router navigation interceptor: a back-gesture
  popstate with unsaved input is vetoed (URL restored) and opens the
  discard sheet instead of leaving. MobileSheet gains pushHistory=false
  so the confirm sheet adds no history sentinel on editor pages — the
  interceptor owns the back gesture there — and discard keeps the
  deterministic replace navigation.
- Draft persistence: create and edit title/description survive a
  reload or app kill via sessionStorage and clear on create, save, and
  discard. The write-back effect retires on exit so it cannot
  resurrect the draft while the page unmounts.
- MobileEditorPage supports a footer snippet (the pinned chip bar);
  sheet transitions run at zero duration under Vitest/jsdom so close()
  stays synchronous for tests.

Author: stefan-ernst