Equip date/time input fields with a "now" option

[Oleg-Melnik]

Closes #140.

Summary

Adds an opt-in "now" affordance to DateTimeField that fills the field with the
current date and time, so the user doesn't have to type it by hand when
recording something at the moment it happens. The current moment is sourced
through WallClock, and the filled value is truncated to the resolution of
dateTimePattern, keeping the stored value consistent with the displayed text.

Changes

• DateTimeField.nowOptionEnabled — opt-in property (off by default) that
  shows a button within the field and enables the Ctrl+N shortcut.
• DateTimeField.nowOptionAffordance — optional custom renderer, receiving a
  fillNow callback, used instead of the default button.
• formatDateTime preserves sub-second precision when the pattern includes
  it; otherwise the value is truncated to the pattern's resolution.
• WallClock.now — exposes the current moment as an Instant, the single
  point through which time-dependent behavior obtains the current time.
• InputField — applyValue sets a value programmatically (reusing the
  normal parse/validate/dirty bookkeeping), and a handleKeyEvent hook lets
  subclasses implement field-specific keyboard shortcuts.
• The default button keeps the field's height, gives hover/press/focus feedback
  and a tooltip, and exposes full button semantics (role, accessible click
  action, disabled state, focusability, and Enter/Space activation).
• Adds compose.materialIconsExtended to the proto module for the button
  icon.
• Tests: WallClockSpec, and DateTimeFieldSpec cases for pattern truncation
  and sub-second round trips.

Reviewer notes

Mouse activation of the default button uses low-level pointer events rather than
clickable/IconButton, because that gesture is intermittently cancelled when
placed inside the text field's decoration slot.