(Dispel Overlay) Fix overlay leaking onto live frames when editing settings in test mode

[Krathe82]

Summary

• UpdateAllDispelOverlays iterates live party/raid frames via IterateAllFrames with no test-mode check
• When called during test mode (e.g. slider commit in the Dispel Overlay settings panel), UpdateDispelOverlay runs the test-mode branch on live secure frames and calls ShowOverlayWithRGB with test data on those hidden live frames
• When test mode closes and live frames become visible, the dispel overlay appears stuck on them — only a /reload clears it
• Fix: UpdateAllDispelOverlays now detects active test mode and routes to the test frame arrays directly, skipping live frames entirely
• When test mode exits (testMode = false is set before the cleanup timer fires), the live-frame cleanup pass runs correctly via the normal IterateAllFrames path

Test plan

• Enter test mode (/df test)
• Enable dispel overlay glow on at least one test frame
• Open Settings → Dispel Overlay, drag and commit any slider (opacity, border, gradient, etc.)
• Close test mode
• Confirm no dispel overlay is stuck on any live frame — no /reload required
• Confirm dispel overlays still appear correctly on live frames when a unit actually has a dispellable debuff

[DanderBot]

Thanks for the PR! Merged into main and will ship in the next alpha build.