feat: tsuki Firefox theme for Nova (157+)

Adaptive light/dark static theme generated from @tsuki/tokens, plus an
optional userChrome.css layer (IBM Plex, tsuki radii, shu accent) that
overrides Nova's design tokens. Self-hosted via AMO unlisted signing and
Gitea releases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assets/icon.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="tsuki">
+  <title>tsuki</title>
+  <rect width="512" height="512" rx="96" ry="96" fill="#9B3C30"/>
+  <text x="256" y="256"
+        font-family="'Noto Serif JP', 'Hiragino Mincho ProN', 'Yu Mincho', 'Songti SC', serif"
+        font-weight="600"
+        font-size="380"
+        text-anchor="middle"
+        dominant-baseline="central"
+        fill="#FAF8F4">月</text>
+</svg>