Fix unit frame health fills breaking on window resize

[robert-clayton]

What does this PR do?

Unit frame health fills render empty after the WoW window is resized, and only recover on a /reload. EllesmereUIUnitFrames registers no display or scale listeners, so a pure resolution change (DISPLAY_SIZE_CHANGED, which fires without a UI-scale change) never triggers a rebuild, and the bars keep their pre-resize pixel-snapped geometry. The main addon's scaleWatcher only re-runs the unit-frame reload through PP.SetUIScale, which a plain resize does not reach.

The fix registers DISPLAY_SIZE_CHANGED + UI_SCALE_CHANGED in the unit-frame addon and re-runs the existing ns.ReloadFrames on those events, deferred one frame so the new screen size settles before the rebuild reads GetPhysicalScreenSize.

How was it tested?

Live (Midnight 12.1) on a Hyprland/Wayland desktop, where Super+F toggles the game window between windowed and fullscreen and fires DISPLAY_SIZE_CHANGED. Before the patch the fills go empty on the toggle and stay empty until /reload; after, they refill correctly on every toggle with no manual reload, in and out of combat.

Screenshots

Before, using Super+F to toggle fullscreen in Hyprland:

After, using Super+F to toggle fullscreen in Hyprland:

Checklist

• New settings default OFF -> N/A. Bug fix, no setting. Criterion 2 exempts genuine bug fixes; the behavior change is broken frames becoming correct frames.
• Zero cost while disabled -> N/A for the same reason. As a bug fix it is always on: one frame and two event registrations. Both events fire only on a resolution/UI-scale change, essentially never during play, so the standing cost is nil.
• Cheap while enabled: event-driven, no polling, no per-frame allocations. Two rare events; the C_Timer.After(0) is a one-shot next-frame defer, not a recurring timer gate.
• No writes onto Blizzard-owned frames. SetScript is on the addon's own CreateFrame("Frame"), never a Blizzard frame. Reuses ns.ReloadFrames, already combat-safe (its throttle guards restricted Show/Hide behind InCombatLockdown and defers via PLAYER_REGEN_ENABLED), so no new taint surface.
• Tested in-game on live; no version gates or pre-Midnight APIs added.