Add regex cheat-sheet icon next to regex inputs

Adds RegexHelpIcon, a hover/click popover (modeled on HelpIcon) showing
a compact cheat sheet for the Rust `regex` crate's actual syntax, and
places it next to every user-facing regex field: Params/Metrics message
match regex, Alerts field regex, and FilterBar's regex(...) helper.

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

webapp/src/components/Metrics.tsx

@@ -12,6 +12,7 @@ import {
 } from '../lib/api'
 import { CollapseToggle, PopOutButton, sectionHeaderClassName, sectionHeaderLeftClassName, useSectionCollapsed } from './CollapsibleSection'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 
 const EMPTY_MATCHER: MetricParamMatcher = { param_name: '', capture_group: '', kind: 'string' }
 
@@ -174,7 +175,10 @@ export function Metrics({ range }: { range: TimeRange | null }) {
             />
           </div>
           <div>
-            <label className="mb-1 block text-xs text-text-subtle">Message match regex</label>
+            <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
+              Message match regex
+              <RegexHelpIcon label="Help: Message match regex syntax" />
+            </label>
             <input
               className="w-full rounded border border-border-soft bg-surface-1 px-2 py-1 font-mono text-sm text-text placeholder:text-text-subtle focus:border-border-strong focus:outline-none"
               value={regex}