Add optional time ranges and a calendar day view

Tasks can now have a start/end time on their deadline date. The new
Calendar view shows a day as a timeline (with a week strip and projected
repeats of recurring tasks), and the task form lists what is already
planned that day and flags overlaps. Existing databases are migrated to
schema v2 automatically, with a backup copy taken first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -2,9 +2,10 @@
 
 A small, self-hosted to-do app for one person. It works in a desktop browser and on a phone, and you can install it to your home screen.
 
-- Tasks have an optional **deadline**, **priority** (none / low / medium / high), **category**, **tags**, **notes**, and a **checklist** of steps.
+- Tasks have an optional **deadline** with an optional **time range** (e.g. 10:00–12:00), **priority** (none / low / medium / high), **category**, **tags**, **notes**, and a **checklist** of steps.
 - **Recurring tasks** (every N days/weeks/months/years): completing one creates the next occurrence.
-- Views: **Today** (overdue, due today, high priority), **Upcoming**, **All open**, **Done**, plus one view per category.
+- When you pick a date and time, the task form lists what else is planned that day and warns about overlaps.
+- Views: **Today** (overdue, due today, high priority), **Upcoming**, **Calendar** (day timeline with week strip; tap an hour to add a task there), **All open**, **Done**, plus one view per category.
 - You can rename, recolor, reorder, add, or delete categories in **Settings**. It starts with Work, Life, Personal, YouTube.
 - **Export / import** of all data as JSON, from **Settings**.
 - Login with a single password.