feat: structures cosmetic effect (hover-shown gradient/transition recolor) (#4492)

## Description:

Adds a new `structures` cosmetic effect type: an equippable effect that
recolors the owner's structure icons (City, Port, Factory, Defense Post,
SAM Launcher, Missile Silo) with gradient or transition color styles.
The effect is **shown while the owner's territory is hovered** —
structures otherwise keep their normal player colors, so the map stays
readable.

**Cosmetics / selection**
- `StructuresEffectAttributesSchema` (`CosmeticSchemas.ts`): its own
discriminated union (`gradient` / `transition`) — structurally identical
to the trail attributes today, but structures aren't trails, so it's a
separate schema free to diverge.
- Slot = the effectType itself: `effectTypeForSlot` is generalized to
map any non-nukeExplosion effect type to itself, so server privilege
checks (`Privilege.ts`), client selection, and persistence all work with
no per-type code.
- Effects tab, Default tile, and the store preview (shared color swatch)
come from `EFFECT_TYPES`; the only UI addition is the
`effects.type.structures` label in `en.json`.

**Rendering**
- The shared per-player effect palette grows from 2 to 3 blocks
(`EFFECT_PALETTE_BLOCKS`; structures = block 2, pinned by a
build-breaking guard). `syncPlayerEffects` resolves the `structures`
selection through the same `writeEffectEntry` used by trails.
- `StructurePass` binds the effect texture plus `uTime` and
`uHoverOwner` (fed from the existing `HoverHighlightController` →
`setHighlightOwner` path, now forwarded to the pass).
- `structure.frag.glsl` recolors the **fill only** — the border keeps
the player color for ownership legibility; alt view and construction
gray bypass the effect entirely.
- Style semantics:
- `gradient` — the palette spans each icon's diagonal once (a visible
gradient across the shape), sliding one full cycle every `colorSize · 4
· count / movementSpeed` seconds (the trail-equivalent pace; world-space
banding like the trail's would put a whole icon inside one band and read
as a flat color)
- `transition` — the whole icon is one color at a time, cross-fading at
`frequency` colors/s
- Glyph contrast: the inner icon's black/white decision is now a smooth
luminance fade (`smoothstep(0.25, 0.45)`) instead of a hard flip at
0.25, so animated fills cross-fade the glyph instead of snapping it
between black and white.

## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/client/components/CosmeticButton.ts

@@ -188,8 +188,9 @@ export class CosmeticButton extends LitElement {
         </div>`;
       }
       // Nuke explosions preview per visual type (expanding ring or sparkle
-      // burst); every trail effectType (transportShipTrail, nukeTrail) shares
-      // the same attributes shape and previews as a color swatch.
+      // burst); every trail effectType (transportShipTrail, nukeTrail) and the
+      // structures effect share the same attribute shapes and preview as a
+      // color swatch.
       if (isNukeExplosionEffect(c)) {
         if (c.attributes.type === "sparkles") {
           return html`<sparkles-swatch

src/client/components/EffectPreview.ts

@@ -2,6 +2,7 @@ import { html, LitElement, TemplateResult } from "lit";
 import { customElement, property } from "lit/decorators.js";
 import {
   NukeExplosionAttributes,
+  StructuresEffectAttributes,
   TrailEffectAttributes,
 } from "../../core/CosmeticSchemas";
 
@@ -9,7 +10,8 @@ import {
 const EMPTY_BG = "#444";
 
 /**
- * Swatch preview of a transport-ship-trail effect, filling its container.
+ * Swatch preview of a trail-styled effect (trails and the structures effect
+ * share the same gradient/transition attribute shapes), filling its container.
  *
  * - gradient / single color: a static swatch (flat color or left-to-right
  *   gradient — a multi-color list reads as a rainbow).
@@ -20,7 +22,7 @@ const EMPTY_BG = "#444";
 export class TrailSwatch extends LitElement {
   // Named `trail` (not `attributes`) to avoid clashing with Element.attributes.
   @property({ attribute: false })
-  trail: TrailEffectAttributes | null = null;
+  trail: TrailEffectAttributes | StructuresEffectAttributes | null = null;
 
   private animation: Animation | null = null;