feat: implement CSS sprite system for image optimization

Reduces HTTP requests from 44+ individual images to 3 sprite sheets
(~93% reduction). Includes Go sprite generator tool, CSS classes,
template integration, and E2E tests.

- Add cmd/sprites/main.go for sprite generation (60x60px + 120x120px @2x)
- Add _sprites.css with responsive sizing and retina support
- Update templates to use sprites with logoIndex fallback
- Add Makefile targets: sprites, sprites-clean
- Add 9-test E2E suite for sprite functionality
- Add doc/22-SPRITES.md with usage documentation

static/images/sprites/sprite-map.json

@@ -0,0 +1,184 @@
+{
+  "companies": [
+    {
+      "index": 0,
+      "name": "accenture.png"
+    },
+    {
+      "index": 1,
+      "name": "aena-long.png"
+    },
+    {
+      "index": 2,
+      "name": "aena.png"
+    },
+    {
+      "index": 3,
+      "name": "clicplan-short.png"
+    },
+    {
+      "index": 4,
+      "name": "clicplan.png"
+    },
+    {
+      "index": 5,
+      "name": "drolosoft.png"
+    },
+    {
+      "index": 6,
+      "name": "drosoloft-plain.png"
+    },
+    {
+      "index": 7,
+      "name": "ebantic.png"
+    },
+    {
+      "index": 8,
+      "name": "emailing-network.png"
+    },
+    {
+      "index": 9,
+      "name": "everis.png"
+    },
+    {
+      "index": 10,
+      "name": "gigya.png"
+    },
+    {
+      "index": 11,
+      "name": "indra.png"
+    },
+    {
+      "index": 12,
+      "name": "insa.png"
+    },
+    {
+      "index": 13,
+      "name": "livgolf.png"
+    },
+    {
+      "index": 14,
+      "name": "megabanner.png"
+    },
+    {
+      "index": 15,
+      "name": "olympic-broadcasting.png"
+    },
+    {
+      "index": 16,
+      "name": "pentamsi-long.png"
+    },
+    {
+      "index": 17,
+      "name": "pentamsi.png"
+    },
+    {
+      "index": 18,
+      "name": "sap.png"
+    },
+    {
+      "index": 19,
+      "name": "twentic.png"
+    },
+    {
+      "index": 20,
+      "name": "uex.png"
+    },
+    {
+      "index": 21,
+      "name": "webratio.png"
+    },
+    {
+      "index": 22,
+      "name": "webratioa.png"
+    }
+  ],
+  "projects": [
+    {
+      "index": 0,
+      "name": "HERRUMBRE_NEGATIVO VER 2@4x.png"
+    },
+    {
+      "index": 1,
+      "name": "deliverybikes.png"
+    },
+    {
+      "index": 2,
+      "name": "herrumbre-vivo.png"
+    },
+    {
+      "index": 3,
+      "name": "jorpack.png"
+    },
+    {
+      "index": 4,
+      "name": "laporra-doc.png"
+    },
+    {
+      "index": 5,
+      "name": "laporra.png"
+    },
+    {
+      "index": 6,
+      "name": "lidering.png"
+    },
+    {
+      "index": 7,
+      "name": "ola-logo-squared-blue.png"
+    },
+    {
+      "index": 8,
+      "name": "sap.png"
+    },
+    {
+      "index": 9,
+      "name": "somosunaola#.png"
+    },
+    {
+      "index": 10,
+      "name": "somosunaola.png"
+    },
+    {
+      "index": 11,
+      "name": "twentic.png"
+    }
+  ],
+  "courses": [
+    {
+      "index": 0,
+      "name": "camaracomercio.png"
+    },
+    {
+      "index": 1,
+      "name": "codecademy.png"
+    },
+    {
+      "index": 2,
+      "name": "forem.png"
+    },
+    {
+      "index": 3,
+      "name": "linkedin-blue.png"
+    },
+    {
+      "index": 4,
+      "name": "linkedin.png"
+    },
+    {
+      "index": 5,
+      "name": "servoy-logo.png"
+    },
+    {
+      "index": 6,
+      "name": "servoy.png"
+    },
+    {
+      "index": 7,
+      "name": "udemy.png"
+    },
+    {
+      "index": 8,
+      "name": "uex.png"
+    }
+  ]
+}