Add white glow behind hovered player's name

The hovered player (tile owner under the cursor, already tracked via
uHighlightOwnerID for cull bypass) now gets a soft white glow behind
their name. The glow is derived from the MSDF distance field: a white
band past the outline with quadratic falloff, composited behind the
glyph and clamped to the SDF margin so it never clips at quad edges.

Glow size and strength are tunable via hoverGlowWidth/hoverGlowAlpha
in render-settings.json, exposed as sliders in the graphics settings
modal (persisted as graphics overrides) and in the debug GUI.

Includes schema and apply tests for the new override fields,
covering the 0 edge case (0 disables the glow, not "unset").

src/client/render/gl/RenderSettings.ts

@@ -243,6 +243,10 @@ export interface RenderSettings {
     statusRowOffset: number;
     /** Alpha multiplier applied to a name while the cursor is over it. */
     hoverFadeAlpha: number;
+    /** White glow behind the hovered player's name: px past the outline. */
+    hoverGlowWidth: number;
+    /** Peak opacity of the hover glow (0 disables it). */
+    hoverGlowAlpha: number;
   };
   fx: {
     shockwaveRingWidth: number;