feat: Add test mode preview for private-auras indicator (GH-#417)

[spamwax]

Summary

This PR adds a test-mode preview for private-auras indicators.

When clicking Highlight Indicator in Grid2 options, the indicator now displays mock private-aura icons with cooldown spirals, timer text, and stack counts. This gives users a more realistic preview of how their private-auras configuration will look before encountering the actual mechanic in combat.

---

Context

I opened issue #417 about previewing private-auras, but the conclusion was that the real Blizzard private-aura frames created through C_UnitAuras.AddPrivateAuraAnchor cannot be directly previewed/customized in the same way, likely because those frames are restricted.

This PR works around that limitation by creating an overlay-style preview instead of trying to drive Blizzard’s actual private-aura frames.

The current test behavior only shows a brief glow/ant-line style highlight, which makes it hard to tune icon size, cooldown display, timer placement, and stack text. Since Blizzard uses private auras for important raid mechanics, being able to configure the indicator before a fight seems useful.

---

Notes

• The preview is intentionally isolated in separate files so it can be maintained, disabled, or removed easily if Blizzard changes the private-aura API.
• The implementation tries to be minimally intrusive to the existing Grid2 codebase.
• This PR was written with AI assistance. Please let me know if that conflicts with the project’s contribution policy or if you prefer changes to be rewritten/restructured.

[michaelnpsp]

Sorry, I dont want to duplicate the blizzard widgets to implemente a test mode. That blizzard code is private/hidden now so it is the blizzard job to implement something like this.
Maybe you can publish this as independent plugin for grid2.
Only one detail, i think this does not work for the indicators > test mode, and the code only supports one private-aura indicator per each frame (but more than one private aura indicator can be created).

[spamwax]

Thanks for the feedback, yea I agree with duplication and that is why I tried to isolate it as much as possible.
are there any sample/resource for plugins?

I did test it by creating an extra private aura indicator, it worked as they were assigned to different mutually exclusive themes! so I did not load 2 PAs at the same time, and your assessment is most likely correct.