dashboard: drop stale data-spw note in side-panel-drawer comment (#451)

argus nit on #456: the CSS comment claimed "The body adds
data-spw=<px> once persisted" — that's leftover from a prior
design and no such attribute is ever set. The actual flow is
localStorage → JS reads on Panel.bind → sets --side-panel-w via
inline style. Replaced the misleading line with an accurate
description of the persistence path.

frontend/packages/dashboard/src/dashboard.css

@@ -1302,10 +1302,10 @@ footer .banner-thin {
   /* #451: width is a CSS variable so the drag handle (added by
      Panel.bind) can update it live, and so localStorage-persisted
      widths apply on first paint. Default min(760px, 94vw) preserves
-     the pre-#451 behaviour for operators who never drag. The body
-     adds `data-spw="<px>"` once persisted; CSS doesn't read it
-     directly — JS sets `--side-panel-w` via inline style on the
-     drawer. */
+     the pre-#451 behaviour for operators who never drag. JS sets
+     `--side-panel-w` via inline style on the drawer; persistence
+     lives in localStorage (key `hyperhive:side-panel-width`),
+     replayed onto the var by `Panel.applyStoredWidth` at bind. */
   width: var(--side-panel-w, min(760px, 94vw));
   /* Clamp so a stored width can never push the drawer off-screen
      or shrink it past readability. min content width matches the