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/Metrics.tsx

@@ -10,6 +10,7 @@ import {
   type MetricDeclaration,
   type MetricParamMatcher,
 } from '../lib/api'
+import { HelpIcon } from './HelpIcon'
 
 const EMPTY_MATCHER: MetricParamMatcher = { param_name: '', capture_group: '', kind: 'string' }
 
@@ -156,7 +157,10 @@ export function Metrics({ range }: { range: TimeRange | null }) {
 
   return (
     <div className="border-b border-border p-3 text-sm">
-      <div className="mb-2 text-xs text-text-subtle">Metrics</div>
+      <div className="mb-2 flex items-center gap-1.5 text-xs text-text-subtle">
+        Metrics
+        <HelpIcon sectionId="metrics" label="Help: Metrics" />
+      </div>
       <div className="flex gap-4">
         <form onSubmit={handleSubmit} className="w-80 shrink-0 space-y-2">
           <div>