feat: add environment filtering across all APIs and UI

Backend: Added optional `environment` query parameter to catalog,
search, stats, timeseries, punchcard, top-errors, logs, and agents
endpoints. ClickHouse queries filter by environment when specified
(literal SQL for AggregatingMergeTree, ? binds for raw tables).
StatsStore interface methods all accept environment parameter.

UI: Added EnvironmentSelector component (compact native select).
LayoutShell extracts distinct environments from agent data and
passes selected environment to catalog and agent queries via URL
search param (?env=). TopBar shows current environment label.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/EnvironmentSelector.module.css

@@ -0,0 +1,26 @@
+.select {
+  appearance: none;
+  background: transparent;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 2px 20px 2px 6px;
+  font-size: 11px;
+  color: var(--text-muted);
+  cursor: pointer;
+  outline: none;
+  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='10' height='6' viewBox='0 0 10 6'%3E%3Cpath d='M1 1l4 4 4-4' stroke='%23888' fill='none' stroke-width='1.5' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E");
+  background-repeat: no-repeat;
+  background-position: right 5px center;
+  background-size: 10px 6px;
+  min-width: 80px;
+  line-height: 1.4;
+}
+
+.select:hover {
+  border-color: var(--text-muted);
+}
+
+.select:focus-visible {
+  border-color: var(--accent);
+  box-shadow: 0 0 0 1px var(--accent);
+}