Calendar: display computed date range in header and align event list

[multiplex55]

Motivation

• Prevent a mismatch between the header label and the events shown by explicitly computing the visible date range per view mode.
• Make the displayed range clear to users by showing start and end dates in the header.
• Reuse the same range calculation for the header and the event list so they always match.
• Ensure range_days is clamped to at least 1 when computing week ranges.

Description

• Added a new helper date_range(&self, selected_date: NaiveDate) -> (NaiveDate, NaiveDate) that computes start/end for Day, Week (using range_days.max(1)), and Month (using add_months) modes.
• Updated render_header() to compute the range and append a "YYYY-MM-DD → YYYY-MM-DD" label to the existing view text.
• Updated render_event_list() to call date_range() and pass start_date/end_date to collect_events() so the list uses the same range as the header.
• Removed duplicated range computation and consolidated logic into the shared helper in src/dashboard/widgets/calendar.rs.

Testing

• No automated tests were run.

---

Codex Task