Make calendar post cards usable with a keyboard

[brudex]

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

Context

The month and week calendar views render previewable posts as clickable div
elements. Mouse users can open a post preview, but these elements are not
keyboard-focusable controls and their icon-only affordances need accessible
names.

Suggested starting points

• public/scripts/app/calendar-month-view.directive.js
• public/scripts/app/calendar-week-view.directive.js
• the calendar styles and existing post-preview handler reached from
  calendarCtrl.openPostPreview(post)

Scope

Replace the mouse-only post-card interaction with native semantic controls or
an equivalent accessible pattern. Preserve the existing click behavior and
visual layout. Add accessible names for icon-only controls in the affected
calendar views when they do not already have a programmatic name.

Non-goals

• Do not redesign the calendar or change scheduling behavior.
• Do not change the post-preview data contract.

Acceptance criteria

• Every interactive post card is reachable by Tab in both month and week
  views.
• Enter and Space open the same preview as mouse activation without creating
  a duplicate action.
• Focus is visible against the existing calendar card styling.
• Icon-only calendar controls have meaningful accessible names.
• Existing mouse preview and add-post interactions continue to work.

Verification

1. Run npm test.
2. Run the app locally with representative scheduled posts.
3. With only a keyboard, navigate to post cards in both calendar views and
   activate each with Enter and Space; confirm the existing preview opens.