feat(desktop): build signed Windows + macOS apps in healthchat pipeline

Extend the HealthChat desktop pipeline beyond Linux to produce fully branded,
signed Windows (.exe/.msi) and macOS (.dmg/.zip) installers, published to the
GitHub Release on the healthchat-v* tag alongside the web build.

- Add HealthChat desktop icons (icon.png/ico + Apple Icon Composer bundle),
  derived from apps/web/res/vector-icons/1024.png
- Point the reusable build workflows at this fork (new repository/ref inputs)
  so the Electron shell, icons and healthchat:// handler are fork-branded
- Relax hardcoded Element*/element-desktop* filename asserts to be
  product-name-agnostic
- Wire signed macOS + Windows jobs into build-healthchat-desktop.yaml, trigger
  it on healthchat-v* tags, and add a release job that uploads installers
- Map the org-level eSigner secrets to the CKA names the reusable expects
- Document signing setup (Apple env, eSigner mapping, ESIGNER_MODE=prod) in the
  desktop README and releasing-healthchat skill

.github/workflows/build_desktop_prepare.yaml

@@ -2,6 +2,14 @@
 on:
     workflow_call:
         inputs:
+            repository:
+                type: string
+                required: false
+                description: "The repository to checkout the desktop source from, defaults to element-hq/element-web"
+            ref:
+                type: string
+                required: false
+                description: "The git ref to checkout, defaults to the default branch"
             config:
                 type: string
                 required: true
@@ -56,7 +64,8 @@ jobs:
             - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
               with:
                   persist-credentials: false
-                  repository: element-hq/element-web
+                  repository: ${{ inputs.repository || 'element-hq/element-web' }}
+                  ref: ${{ inputs.ref }}
 
             - uses: pnpm/action-setup@0e279bb959325dab635dd2c09392533439d90093 # v6.0.8
             - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6