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>

test/models/application_test.rb

@@ -29,4 +29,31 @@ class ApplicationTest < ActiveSupport::TestCase
     tempfile&.close
     tempfile&.unlink
   end
+
+  test "icon_dark is independently attachable and SVG-sanitized" do
+    app = applications(:kavita_app)
+
+    svg = %(<svg xmlns="http://www.w3.org/2000/svg"><script>boom()</script><circle cx="5" cy="5" r="3"/></svg>)
+    tempfile = Tempfile.new(["dark", ".svg"]).tap do |t|
+      t.write(svg)
+      t.rewind
+    end
+    uploaded = ActionDispatch::Http::UploadedFile.new(
+      tempfile: tempfile,
+      filename: "dark.svg",
+      type: "image/svg+xml"
+    )
+
+    assert_nothing_raised do
+      app.update!(icon_dark: uploaded)
+    end
+
+    assert app.icon_dark.attached?
+    cleaned = app.icon_dark.download
+    refute_match(/<script/i, cleaned)
+    assert_match(/<circle/, cleaned)
+  ensure
+    tempfile&.close
+    tempfile&.unlink
+  end
 end