ui: the block route is a page too

Same reasoning as the miner and account routes one commit ago, which the block
route was deliberately left out of: a block is read against the standings, and
the ticker row below is what you clicked to get here. On reflection that is an
argument about how someone arrived, not about what the URL names — and the URL
names one block. The board is what `/:chain/:window` is for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jp6a8EDar9ueEhAxzep4V5

web/src/components/BlockPanel.tsx

@@ -56,7 +56,6 @@ export function BlockPanel({
   block,
   targetSeconds,
   onResolved,
-  onClose,
 }: {
   chain: string
   /** Height or hash, as it appeared in the URL. */
@@ -65,7 +64,6 @@ export function BlockPanel({
   targetSeconds: number
   /** Called with the canonical hash once a height has resolved to one. */
   onResolved: (hash: string) => void
-  onClose: () => void
 }) {
   const [detail, setDetail] = useState<BlockDetail | null>(null)
   const [error, setError] = useState<string | null>(null)
@@ -122,21 +120,9 @@ export function BlockPanel({
             {detail?.hash ?? ''}
           </div>
         </div>
-        <button
-          className="segmented"
-          style={{
-            padding: '7px 13px',
-            font: 'inherit',
-            fontSize: 12,
-            letterSpacing: '0.1em',
-            textTransform: 'uppercase',
-            color: 'var(--text-secondary)',
-            cursor: 'pointer',
-          }}
-          onClick={onClose}
-        >
-          Close
-        </button>
+        {/* No close button: this is a URL somebody navigated to, not a dialog
+            that opened over something they were reading. The back button and
+            the chain chip both already do what it would. */}
       </div>
 
       {error && <p className="empty">{error}</p>}