Show deadlines in the Schedule view

Problem Statement

The Schedule view has no deadline awareness at all. src/app/features/schedule/ contains zero references to deadlines, so:

• a task with a deadline but no "planned for" date is completely invisible in Schedule
• a task with both shows nothing to indicate that a deadline exists or when it falls

The Planner already handles this. groupDeadlineTasksByDay() in planner.selectors.ts buckets each undone deadline task onto its deadline day, rendered via planner-deadline-task behind a flag icon (planner-day.component.html).

That asymmetry between the two planning surfaces is, I think, what #9484 and #8211 were actually reacting to, and it is a much smaller problem than the multi-day spanning both of them proposed.

Possible Solution

A deadline marker on the deadline day in the Schedule week and month views.

A point marker, not a span: the days between "planned for" and "deadline" are slack, not occupancy, and drawing a bar across them would tell the user the task consumes time it doesn't. See #9553 for the longer version of that reasoning.

There is a good pattern to reuse. The over-budget badge in the week header (.over-budget-badge, schedule-week.component.html + schedule-week.component.scss) is already a small pill with a symbol, an optional count and a tooltip, and it costs no grid space. Swapping the ! for the flag icon used elsewhere for deadlines would fit the existing visual vocabulary.

Additional context

• No new field and no schema change. deadlineDay / deadlineWithTime already exist.
• Should be calm by default: a quiet marker, nothing attention-grabbing, no new setting.
• Prior art: Things 3 has both a start date and a deadline, and still does not derive a range from them. Its docs draw the same distinction we do ("Contrary to start dates, which describe when you want to begin working on a task, deadlines define the date by which you must finish the to-do"), and where a task needs several days it tells the user to "add a start date several days before the deadline" rather than inferring one.

Related

• #9484, #8211: multi-day spanning. Different concept, still open in #8211.
• #6078, #4647: start dates.
• #9553: the declined ghost-bar approach, with the full reasoning.