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/lib/docsNav.tsx

@@ -0,0 +1,35 @@
+import { createContext, useCallback, useContext, useMemo, useRef, useState, type ReactNode } from 'react'
+
+/** One request to open the Docs panel scrolled to a specific section (ticket 021). */
+export interface DocsTarget {
+  sectionId: string
+  /** Bumped on every request, even to the same section, so a repeat click re-triggers the flash. */
+  nonce: number
+}
+
+interface DocsNavValue {
+  target: DocsTarget | null
+  openSection: (sectionId: string) => void
+}
+
+const DocsNavContext = createContext<DocsNavValue | null>(null)
+
+/** Wraps the app so any component can open Docs to a specific section via `useDocsNav`. */
+export function DocsNavProvider({ children }: { children: ReactNode }) {
+  const [target, setTarget] = useState<DocsTarget | null>(null)
+  const nonceRef = useRef(0)
+
+  const openSection = useCallback((sectionId: string) => {
+    nonceRef.current += 1
+    setTarget({ sectionId, nonce: nonceRef.current })
+  }, [])
+
+  const value = useMemo(() => ({ target, openSection }), [target, openSection])
+  return <DocsNavContext.Provider value={value}>{children}</DocsNavContext.Provider>
+}
+
+export function useDocsNav(): DocsNavValue {
+  const ctx = useContext(DocsNavContext)
+  if (!ctx) throw new Error('useDocsNav must be used within a DocsNavProvider')
+  return ctx
+}