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/Alerts.tsx

@@ -11,6 +11,7 @@ import {
 } from '../lib/api'
 import { sectionHeaderClassName, sectionHeaderLeftClassName } from './CollapsibleSection'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 import { queryInputClassName } from './FilterBar'
 
 type ConditionKind = AlertCondition['kind']
@@ -323,7 +324,10 @@ export function Alerts() {
           {form.kind === 'newvalue' && (
             <div className="space-y-1">
               <div>
-                <label className="mb-1 block text-xs text-text-subtle">Field regex</label>
+                <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
+                  Field regex
+                  <RegexHelpIcon label="Help: Field 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={form.fieldRegex}