Add number-box interval input, parallel coordinator refresh, and changelog entry

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Motivation

• Improve the user experience when configuring the polling interval by providing a numeric "number box" input instead of a plain integer field.
• Reduce startup latency when multiple servers are configured by performing the coordinators' initial refreshes concurrently.
• Ensure container and disk entities are created from fresh coordinator data so entity discovery is correct after the initial refresh.
• Document the user-visible changes in the release notes so users are informed of the update.

Description

• Replaced plain integer interval fields with selector.NumberSelector (box mode, min=1, step=1) in the server schema and the options flow init form in config_flow.py.
• Collected VServerCoordinator instances and invoked their async_config_entry_first_refresh concurrently via asyncio.gather in sensor.py to parallelize initial data fetch.
• Deferred creation of container and disk entities until after the concurrent refresh by storing ServerContainerRegistry/ServerDiskRegistry tuples and creating initial entities from the coordinators' refreshed data.
• Added a v1.2.24 entry to CHANGELOG.md describing the interval input change and the parallel refresh improvement.

Testing

• No automated tests were run as part of this change.
• Basic repository operations (adding and committing CHANGELOG.md) were performed successfully during the rollout.
• Manual validation of code formatting and imports was not recorded as automated tests.
• No test failures were reported.

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Codex Task