docs: add web UI scaffold design spec

First pass of the web UI design doc from the M1 close-out brainstorm.
Covers the stack (SvelteKit static + Go embed), auth model
(cookie + bearer from one endpoint), /api/* surface, shell layout
(sidebar + bottom player), first-cut feature scope, and how later
web UI work threads into M2–M5 alongside each backend milestone
instead of landing as a post-hoc M6.

Also ignores the local .superpowers/ brainstorming companion cache.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,6 @@ go.work.sum
 # env file
 .env
 
+
+# Superpowers brainstorming companion sessions
+.superpowers/