refactor(theme): single-source the HealthChat Light theme

Author the theme once in apps/web/element.io/healthchat/HealthChat-Light-Theme.json
and sync it into the web (apps/web/config.json) and desktop
(apps/desktop/element.io/healthchat/config.json) configs via
scripts/healthchat-sync-theme.mjs, removing the drifted duplicate.
`pnpm healthchat:theme` writes it; `pnpm lint` runs healthchat:theme:check to
fail on drift.

The canonical palette is the reviewed desktop theme, which reverts the web-only
contrast tweak (f64923ea1a) so web and desktop match.

apps/web/element.io/healthchat/HealthChat-Light-Theme.json

@@ -0,0 +1,41 @@
+{
+    "name": "HealthChat Light",
+    "is_dark": false,
+    "colors": {
+        "accent-color": "#1f4d85",
+        "primary-color": "#1f4d85",
+        "alert": "#df8e1d",
+        "warning-color": "#d20f39",
+        "sidebar-color": "#12325c",
+        "roomlist-background-color": "#f5f5f5",
+        "roomlist-highlights-color": "#d9d9d9",
+        "roomlist-separator-color": "#9b9898",
+        "roomlist-text-color": "#473e3e",
+        "roomlist-text-secondary-color": "#9b9898",
+        "timeline-background-color": "#ffffff",
+        "timeline-highlights-color": "#d9d9d9",
+        "timeline-text-color": "#473e3e",
+        "timeline-text-secondary-color": "#9b9898",
+        "secondary-content": "#9b9898",
+        "tertiary-content": "#9b9898",
+        "focus-bg-color": "#d9d9d9",
+        "menu-selected-color": "#d9d9d9",
+        "other-user-pill-bg-color": "#376A91",
+        "room-highlight-color": "#376A91",
+        "reaction-row-button-selected-bg-color": "#d9d9d9",
+        "togglesw-off-color": "#9b9898",
+        "avatar-background-colors": ["#12325c", "#1f4d85", "#376A91"],
+        "username-colors": ["#12325c", "#2a4063", "#1f4d85", "#376A91", "#473e3e", "#1f4d85", "#2a4063", "#12325c"]
+    },
+    "fonts": {
+        "general": "\"Century Gothic\", \"Futura\", \"Trebuchet MS\", \"Arial\", sans-serif"
+    },
+    "compound": {
+        "--cpd-color-text-action-accent": "#1f4d85",
+        "--cpd-color-bg-action-primary-rest": "#1f4d85",
+        "--cpd-color-bg-action-primary-hovered": "#2a4063",
+        "--cpd-color-bg-action-primary-pressed": "#12325c",
+        "--cpd-color-text-link-external": "#1f4d85",
+        "--cpd-color-icon-accent-tertiary": "#376A91"
+    }
+}

apps/web/element.io/healthchat/README

@@ -0,0 +1,25 @@
+HealthChat theme — single source of truth
+=========================================
+
+  HealthChat-Light-Theme.json   The canonical "HealthChat Light" theme.
+
+Edit the palette HERE and nowhere else. apps/web/scripts/healthchat-sync-theme.mjs injects
+it into the element-web configs that ship it:
+
+  - apps/web/config.json                            (dev server; also baked into the web image)
+  - apps/desktop/element.io/healthchat/config.json  (desktop variant, bundled into the app)
+
+After editing, run:
+  pnpm --filter element-web healthchat:theme
+`pnpm lint` runs `healthchat:theme:check`, which fails if a config is out of sync — so an
+un-propagated edit is caught in CI.
+
+Production web deployment
+-------------------------
+The production web client is deployed by matrix-docker-ansible-deploy, whose
+matrix-client-element role renders config.json from host_vars (no upstream role change).
+It carries an INLINE copy of this theme in
+  inventory/host_vars/matrix.healthchat.ch/vars.yml
+    (matrix_client_element_setting_defaults_custom_themes)
+When you change the theme here, re-copy it into that host_vars block. themes_enabled is off
+there, so the (stale) Novaloop-AG/element-themes repo does not append an older theme.