feat: add Default button to DF colour picker

[Krathe82]

Summary

• Adds a Default button to the DF colour picker, stacked below the Save button in the tab row. Clicking it resets the colour to the setting's configured default value with live preview.
• Populates ElvUI's ColorPPDefault button with the DF default when any DF setting's colour picker is opened, enabling it on the native Blizzard picker.
• Resource bar (power) colour pickers now correctly resolve defaults via WoW's PowerBarColor table as a fallback when no entry exists in PartyDefaults/RaidDefaults.
• Fixes a noisy "Color picker hook already active" chat message that fired incorrectly when toggling the global color picker override checkbox.

Test plan

• Open any DF colour setting — Default button appears below Save in the picker tab row and resets to the setting default with live preview
• Open a resource bar power colour — Default button appears and resets to the WoW default for that power type
• With ElvUI installed and DF override off, open a DF colour setting — ElvUI's Default button on the native picker is enabled and pastes the correct DF default
• Toggle "Use DF Color Picker for All Addons" on and off — no spurious chat messages