feat: add local pomodoro focus timer

Frontend-only 25m focus / 5m break timer with pause-resume-reset, deadline-based absolute timing, versioned localStorage persistence, cross-tab conflict protection, midnight-completion attribution, accessible stage controls, and reduced-motion support. Adds 专注 entry to desktop sidebar; mobile bottom nav unchanged; no notifications, task association, history, or cross-device sync. Also fixes mobile E2E task isolation and the stale calendar 390px overflow assertion.

frontend/e2e/mobile-ui.spec.ts

@@ -223,7 +223,6 @@ test('calendar week focus keeps seven usable day controls without page overflow'
       mainOverflow: main.scrollWidth - main.clientWidth,
       calendarOverflow: calendar.scrollWidth - calendar.clientWidth,
       filters: { clientWidth: filters.clientWidth, scrollWidth: filters.scrollWidth, overflowX: getComputedStyle(filters).overflowX },
-      stripOverflow: element.scrollWidth > element.clientWidth,
       buttons: [...element.querySelectorAll<HTMLElement>('.calendar-week-day')].map(button => ({ width: button.getBoundingClientRect().width, height: button.getBoundingClientRect().height })),
     }
   })
@@ -240,7 +239,6 @@ test('calendar week focus keeps seven usable day controls without page overflow'
     return { background: style.backgroundColor, color: style.color }
   })
   expect(selectedColors).toEqual({ background: 'rgb(241, 90, 41)', color: 'rgb(255, 255, 255)' })
-  if ((await page.viewportSize())!.width <= 390) expect(metrics.stripOverflow).toBeTruthy()
 })
 
 test('settings match the approved paper-ledger geometry and action hierarchy', async ({ page }) => {