Add inline format editor for clock and countdown

[danielchalmers]

Replaces the raw format controls with a bespoke inline editor so customizing the clock and countdown is approachable for common scenarios without losing the free-form escape hatch.

A reusable FormatEditor control (with a Clock/Countdown mode) is embedded directly in the Display and Countdown cards — no dialog — and is organized into labeled sections:

• Common formats — one-click preset chips that show their live rendered output (e.g. "Sortable · 2026-07-06 13:16"), so users pick by result instead of decoding format strings. Countdown presets include "Automatic", which blanks the format and replaces the old Reset button.
• Custom format — the familiar free-form text box, now with an "Insert:" row of building-block chips (Weekday, Day, Month, Year, Time, Seconds, UTC offset / Days, Hours, Minutes, Seconds) that insert their token at the cursor.
• Preview — a live result panel that ticks every second, renders through the same TimeStringFormatter path as the clock itself, and turns red on invalid formats. With no countdown target set, previews use a sample target 3 days out so the chips aren't empty.

Edits bind straight to settings (UpdateSourceTrigger=PropertyChanged), so the desktop clock updates as you type — the app itself is the final preview.

The countdown target gets the same treatment via a CountdownTargetEditor: one-click preset targets (Midnight tonight, Tomorrow morning, Friday 5 PM, New Year's) computed at click time and labeled with the actual date they resolve to, the free-form box with Reset, and a plain-words preview that humanizes the target ("Friday, July 10, 2026 5:00 PM — 3 days from now") or states that countdown mode is off.

Removed

• The examples ComboBox next to the clock format box (presets supersede it).
• The countdown format Reset button (superseded by the "Automatic" preset).
• The intermediate FormatEditorWindow dialog from the first prototype commit — feedback moved the editor inline.

Notes

• The clock format keeps 12-hour presets alongside explicit 24-hour variants; both token sets avoid single-character tokens ({%d} etc. in countdown mode) so they survive the standard-format-string interpretation.
• Verified by rendering the real SettingsWindow headlessly under the dark palette; 78/78 tests pass and dotnet format is clean.
• DateFormatExample.DefaultExamples is no longer referenced from XAML but is kept since tests cover it; can drop it in a follow-up if preferred.

Before/after