Feature: Add Fan Operating Mode select for active fan mode

[EdenNelson]

ESPHome Web UI does not expose Home Assistant fan presets/modes as clearly as a normal select entity. Add a Levoit fan_operating_mode select that exposes the purifier's active fan operating mode directly.

This gives ESPHome dashboards a regular dropdown for the same active fan modes already supported by the fan preset path, while keeping existing fan preset behavior intact.

Acceptance criteria:

• Add a new Levoit select type with YAML config key type: fan_operating_mode.
• The internal select enum uses a unique name and does not conflict with existing text sensor or fan mode names.
• The select exposes model-specific active fan modes:
  • Core200S: Manual, Sleep
  • Core300S/Core400S/Core600S: Manual, Sleep, Auto
  • Vital100S/Vital200S: Manual, Sleep, Auto, Pet
  • Sprout: Manual, Auto
  • EverestAir: Manual, Sleep, Auto, Turbo
• Changing the select sends the same MCU fan-mode commands already used by the fan preset path.
• MCU status updates publish back to the select so it stays synchronized with physical-panel and Home Assistant changes.
• Local fan-control changes also update the select immediately, so the dropdown stays in sync when fan operation changes.
• Add the select to relevant default Levoit device YAMLs and builder YAML mirrors.
• Document that Fan Operating Mode is the active fan mode and is distinct from Auto Mode / Auto Profile.
• Do not add remembered Room Size persistence, Manual/Auto mechanics changes, Core room-size encoding changes, diagnostics, raw sensors, generated YAML churn beyond required config mirrors, or unrelated model behavior changes.

[tuct]

@EdenNelson Can you check the PR? There is a small merge conflict