Add an accessible input contract to datepicker-input

[brudex]

Proposed labels: accessibility, area:frontend, good first issue

Context

The reusable datepicker-input directive creates a readonly text input, but
callers cannot reliably supply an input ID or connect a visible label and
description. Its popup state is also not communicated to assistive technology.

Suggested starting points

• public/scripts/app/datepicker-input.directive.js
• templates that render <datepicker-input>, including post-editor views
• the bundled datepicker's show/hide lifecycle already handled by the
  directive

Scope

Add optional directive attributes for an input ID and label/description wiring.
Expose the datepicker's expanded state and popup relationship using suitable
ARIA attributes, keeping the existing Angular model and date format behavior.

Non-goals

• Do not replace or upgrade the datepicker library.
• Do not change date serialization, min/max-date semantics, or default popup
  placement.

Acceptance criteria

• A caller can give the generated input a stable ID and associate it with a
  label and optional description.
• The input communicates that it opens a date dialog/popup and whether it is
  expanded.
• Opening, selecting a date, pressing Escape, and hiding the picker leave the
  ARIA state accurate.
• Existing ngModel, min/max dates, and on-change behavior remain intact.

Verification

1. Run npm test.
2. Exercise an existing post-editor date picker with a keyboard: focus, open,
   select a date, press Escape, and confirm the value and popup state update.
3. Inspect the rendered input in browser developer tools to confirm label and
   ARIA relationships are present when attributes are supplied.