fix(dashboard): gate overlay never hides due to missing [hidden] CSS rule

.gate had display:flex with no [hidden] override, so the auth gate
rendered on top of the dashboard permanently regardless of token
validity — every sibling gated element in this file (.menu, .badge,
.panel, .segbadge, .view, .live-health, .live-empty, .sdetail) already
follows this pattern; .gate was the one omission (from PR #68).

Verified with Playwright: before the fix, computed display was "flex"
even with the hidden attribute set (token accepted); after, "none".

Author: pacphi

src/lib/dashboard/styles.mjs

@@ -49,6 +49,7 @@ body{
 /* ── token gate (dashboard requires a per-session token, same contract as
    admin — ADR-0014) — hides the rest of the page via body.gated below ── */
 .gate{position:fixed;inset:0;z-index:50;display:flex;align-items:center;justify-content:center;background:var(--bg);padding:24px}
+.gate[hidden]{display:none}
 .gate-card{max-width:460px;background:var(--panel);border:1px solid var(--line);border-radius:var(--r);padding:26px 28px}
 .gate-card p{font-size:14px;color:var(--ink-2);margin:0 0 14px;line-height:1.5}
 .gate-card code{font-family:var(--mono);color:var(--accent);font-size:12.5px}