Generated monogram fallback + optional dark-mode icon per application

When an application has no icon attached, render a deterministic
monogram SVG instead of the generic picture-frame placeholder. Initials
are picked from capital letters in the name (ShelfLife -> SL); fall
back to the first two letters when fewer than two capitals exist
(Audiobookshelf -> AU). Background colour is hashed from the name for
stable per-app identity across visits.

Adds an optional second icon attachment, icon_dark, alongside the main
icon. When present, render a <picture> with a prefers-color-scheme:
dark source so the browser swaps automatically; when absent, the main
icon is used in both modes. The SVG sanitization, content-type fix,
and size/format validation now run over both attachments uniformly.

Bumps to 0.14.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/controllers/admin/applications_controller.rb

@@ -110,7 +110,7 @@ module Admin
       permitted = params.require(:application).permit(
         :name, :slug, :app_type, :active, :redirect_uris, :description, :metadata,
         :domain_pattern, :landing_url, :access_token_ttl, :refresh_token_ttl, :id_token_ttl,
-        :icon, :backchannel_logout_uri, :is_public_client, :require_pkce, :skip_consent
+        :icon, :icon_dark, :backchannel_logout_uri, :is_public_client, :require_pkce, :skip_consent
       )
 
       # Handle headers_config - it comes as a JSON string from the text area