UX: keep events visible while loading; add subtle corner loader; add aria-busy on card

[IT-BAER]

Description

• Replace full-card loading text state with a small non-blocking spinner in the card’s top-right corner.
• Keep existing content visible while data refreshes to avoid flicker and layout shift.
• Add aria-busy on ha-card and role=status on the indicator for better accessibility.
• Preserve DOM structure for card-mod compatibility.

Changes highlight:

• render.ts: add isLoading flag to renderMainCardStructure, render a .loading-indicator spinner, set aria-busy on ha-card.
• Styles: add .loading-indicator/.spinner classes (small 14px spinner, pointer-events: none).

Related Issue

This PR fixes or closes issue: fixes #

Motivation and Context

Improves UX by avoiding replacing all events with loading text, reduces layout shifts, and provides accessible status feedback while loading.

How Has This Been Tested

• Built local bundle and copied to Home Assistant www as a dashboard resource.
• Used type: custom:calendar-card-pro-dev with refresh_interval: 1 to observe frequent refresh.
• Verified:
  • Existing events remain visible during loads.
  • Small spinner appears in the top-right corner and does not block interactions.
  • aria-busy toggles correctly; indicator uses role=status and aria-live=polite.
  • Empty-state and normal-state behave consistently.
• Environment: Home Assistant frontend in browser on Windows.

Types of changes

• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🚀 New feature (non-breaking change which adds functionality)
• 🌎 Translation (addition or update for a language)
• ⚙️ Tech (code style improvement, performance improvement or dependencies update)
• 📚 Documentation (fix or addition to documentation)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have linted and formatted my code (npm run lint and npm run format)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have tested the change in my local Home Assistant instance.
• I have followed the translation guidelines if I'm adding or updating a translation.