Bring offline mode to full parity with the online dashboard

Fixes subtasks appearing flattened into the main task list (missing
parent filter), and adds everything else needed for full offline
functionality: tag chips/assignment/creation, full field editing
(description, due time, recurrence presets), time tracking with a
live-updating timer, and properly nested subtasks -- all queued
locally and synced via the existing /api/sync/ protocol.

The offline page now reuses the real dashboard's app-layout/header/
sidebar/detail-pane structure instead of a bespoke layout, with an
amber-tinted header and banner so it's unmistakable which mode you're
in. The detail panel mirrors _task_detail.html's fields and actions.

Also fixes a real backend gap this feature depends on: apply_conflict_data()
in sync/views.py silently no-op'd on time_entry conflicts. And computes
duration_seconds client-side on merge, since TimeEntrySyncSerializer
doesn't include it in the wire format at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/sw.js

@@ -1,4 +1,4 @@
-{% load static %}const CACHE_NAME = 'keepitgoing-shell-v3';
+{% load static %}const CACHE_NAME = 'keepitgoing-shell-v5';
 const OFFLINE_URL = '{% url "offline" %}';
 const OFFLINE_TASKS_URL = '{% url "offline-tasks" %}';