ui: migrate to shadcn/ui primitives across dialogs, filters, and forms

Adopts shadcn/ui components (Dialog, Button, Input, Select, Popover,
Command, Checkbox, Switch, Toggle, Calendar, etc.) across the app,
replacing hand-rolled modals, dropdowns, and form controls. Adds a
reusable cmdk-backed MultiSelect for the Type, Tags, and Flag filters
so all multi-value filter popovers share one component and layout.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/input.tsx

@@ -0,0 +1,23 @@
+import * as React from 'react'
+
+import { cn } from '@/lib/utils'
+
+const Input = React.forwardRef<
+  HTMLInputElement,
+  React.InputHTMLAttributes<HTMLInputElement>
+>(({ className, type, ...props }, ref) => {
+  return (
+    <input
+      type={type}
+      ref={ref}
+      className={cn(
+        'flex h-8 w-full rounded-md border border-border bg-surface-offset px-2.5 text-sm text-text placeholder:text-text-faint focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-primary disabled:cursor-not-allowed disabled:opacity-50 file:border-0 file:bg-transparent file:text-sm file:font-medium',
+        className
+      )}
+      {...props}
+    />
+  )
+})
+Input.displayName = 'Input'
+
+export { Input }