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/LogTimeline.tsx

@@ -1,5 +1,6 @@
 import { useRef, useState } from 'react'
 import type { TimelineBucket } from '../lib/api'
+import { HelpIcon } from './HelpIcon'
 
 export interface TimeRange {
   startMs: number
@@ -59,7 +60,10 @@ export function LogTimeline({
   return (
     <div className="border-b border-border p-3">
       <div className="mb-1 flex items-center justify-between text-xs text-text-subtle">
-        <span>Timeline</span>
+        <span className="flex items-center gap-1.5">
+          Timeline
+          <HelpIcon sectionId="timeline" label="Help: Timeline" />
+        </span>
         {range && (
           <button className="underline hover:text-text-secondary" onClick={() => onSelectRange(null)}>
             Clear selection