Add pop-out button to open a section in its own tab (ticket 036)

Each of the four primary sections (Log viewer, Params, Metrics,
Dashboards) gets a PopOutButton next to its CollapseToggle/HelpIcon that
opens `?section=<id>` in a new tab via window.open. App.tsx branches on
that query param to render just the one section full-page, with no
NavMenu/FilterBar/sibling chrome — Metrics/Dashboards get the range that
was active at pop-out time via startMs/endMs query params (they have no
LogTimeline of their own to pick one), Params gets a small self-contained
params fetch since it has no LogPreview sibling to share state with, and
the popped-out Log viewer owns its own copy of the log/timeline polling
state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/CollapsibleSection.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react'
+import type { TimeRange } from './LogTimeline'
 
 /// localStorage key for which of the four primary sections (log viewer,
 /// params, metrics, dashboards) are collapsed (ticket 035). Mirrors
@@ -57,3 +58,34 @@ export function CollapseToggle({ collapsed, onToggle }: { collapsed: boolean; on
     </button>
   )
 }
+
+/// The four primary sections that can be popped out into their own tab
+/// (ticket 036) — matches the `?section=` query param a standalone page reads.
+export type PopOutSectionId = 'log' | 'params' | 'metrics' | 'dashboards'
+
+function popOutUrl(section: PopOutSectionId, range?: TimeRange | null): string {
+  const query = new URLSearchParams({ section })
+  if (range) {
+    query.set('startMs', String(range.startMs))
+    query.set('endMs', String(range.endMs))
+  }
+  return `?${query.toString()}`
+}
+
+/// Opens this section, full-page and with no surrounding chrome, in a new
+/// tab. `range` is only needed for sections without their own LogTimeline —
+/// the popped-out page has no way to pick a range itself, so the current one
+/// is carried over via query params (see `App.tsx`'s standalone rendering).
+export function PopOutButton({ section, range }: { section: PopOutSectionId; range?: TimeRange | null }) {
+  return (
+    <button
+      type="button"
+      onClick={() => window.open(popOutUrl(section, range), '_blank')}
+      title="Open in new tab"
+      aria-label="Open in new tab"
+      className="flex h-4 w-4 shrink-0 items-center justify-center rounded-full border border-border-soft text-[10px] leading-none text-text-faint hover:border-border-strong hover:text-text-secondary"
+    >
+      ↗
+    </button>
+  )
+}