The look from the server's tokens, pinned; the frame and bar follow the page's theme

ui/tokens.css is no longer a hand copy that drifts: scripts/fetch-tokens.sh
takes the server's tokens.css at a git ref and tokens.lock.json pins it, as
fetch-cli.sh does for the CLI. build.rs reads the two themes' --bg and --border
out of it, so no colour is written in Rust. The content view reports the page's
theme by a navigation the app refuses (llembas-theme:<name>), and the window,
the title bar and the first-run page are painted in it and remember it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tokens.lock.json

@@ -0,0 +1,5 @@
+{
+  "tag": "HEAD",
+  "commit": "bae579b69182554a2677b5617f11c11decd280db",
+  "sha256": "a6ad29c609dca02430edf6cd67c30bc8e5bdd1d23f6dc275a8b3d764e6254d52"
+}