feat: Update app logo and remove unused favicon (#1315)

## Problem

App icons needed refreshing with the new logo and the favicon-dark SVG was unused dead weight.

![CleanShot 2026-03-23 at 12.05.26@2x.png](https://app.graphite.com/user-attachments/assets/3db855d3-82be-4816-9593-403dd6865ed6.png)

## Changes

1. Replace app icon source PNGs with updated logo
2. Regenerate macOS .icns and Windows .ico from new source
3. Update liquid glass icon config and asset (icon.png → logo.png)
4. Add high-res icon@3x.png and logo.png source files
5. Remove unused posthog-code-favicon-dark.svg

## How did you test this?

Manually

Author: charlesvien

apps/code/build/app-icon.icns

@@ -1,24 +1,23 @@
 {
   "fill": {
-    "automatic-gradient": "display-p3:0.00000,0.00000,0.00000,1.00000"
+    "solid": "display-p3:0.97647,0.93333,0.88235,1.00000"
   },
   "groups": [
     {
+      "blur-material": null,
       "layers": [
         {
-          "fill": "none",
-          "image-name": "icon.png",
-          "name": "icon",
-          "position": {
-            "scale": 0.4,
-            "translation-in-points": [3, 3]
-          }
+          "glass": false,
+          "image-name": "logo.png",
+          "name": "logo"
         }
       ],
+      "lighting": "individual",
       "shadow": {
         "kind": "neutral",
         "opacity": 0.5
       },
+      "specular": true,
       "translucency": {
         "enabled": true,
         "value": 0.5