feat: add native update support and improve agent discovery logic

[RF1705]

Title: Fix single-agent discovery and use native update entities

Summary

This PR fixes the Dockge Card discovery logic for single-agent setups and adds support for Home Assistant’s native update.install flow.

Fixes #1

What changed

• Fixed card discovery when no Global Summary entity exists.
• Added fallback discovery from Dockge container sensors.
• Kept existing multi-agent discovery via Global Summary.
• Prevented single-agent setups from sending agent: "Primary" to Dockge services.
• Added support for native Home Assistant update entities.
• The Update popup action now calls update.install when a matching update.* entity exists.
• Falls back to dockge.update_stack for older Dockge integration versions.
• Improved update count detection from:
  • sensor.*image_updates_available
  • update.* entities
  • container update_available attributes
• Improved processing detection to include both binary sensors and update entities.
• Bumped card version to 1.6.1.
• Updated README cache-busting example and feature list.

Why

In single-agent installations, the Dockge HA integration does not create a Global Summary sensor. The card previously required that entity and therefore displayed:

No Dockge integration found.

This PR allows the card to discover stacks and containers directly from existing Dockge container sensors, so it works in both single-agent and multi-agent setups.

Testing

Validated JavaScript syntax with:

node --check dockge-card.js