Add contextual "?" help icons linking into Docs (ticket 021)

New HelpIcon component + a DocsNavProvider context (webapp/src/lib/docsNav.tsx)
let any component request "open Docs to section X", bumping a nonce each
time so a repeat click re-triggers even if Docs is already open to that
section. NavMenu forwards that request into DocsPanel as a `target` prop,
which expands the right accordion section, scrolls it into view, and
flashes it (a brief accent-tinted background/border, settling back after
1.2s) — verified visually with a live server + Playwright. Opening Docs via
the plain NavMenu button explicitly clears any pending target, so that path
is unchanged: no forced section, no flash.

Added a "?" icon to FilterBar, LogTimeline, LogPreview, Metrics, Dashboards,
and NavMenu's own Settings/Presets panels, plus new Docs sections for
Timeline, Log preview, Settings, and Presets (previously undocumented).

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

webapp/src/components/HelpIcon.tsx

@@ -0,0 +1,21 @@
+import { useDocsNav } from '../lib/docsNav'
+
+/**
+ * A small "?" affordance that opens the Docs panel scrolled to (and
+ * flashing) a specific section (ticket 021), instead of the user having to
+ * find it themselves in the Docs accordion.
+ */
+export function HelpIcon({ sectionId, label }: { sectionId: string; label?: string }) {
+  const { openSection } = useDocsNav()
+  return (
+    <button
+      type="button"
+      onClick={() => openSection(sectionId)}
+      title="Open docs"
+      aria-label={label ?? 'Open docs for this section'}
+      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>
+  )
+}