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/lib/utils.ts

@@ -0,0 +1,9 @@
+import { clsx, type ClassValue } from 'clsx'
+import { twMerge } from 'tailwind-merge'
+
+/** Tailwind-aware className combiner. Used by every shadcn ui/*.tsx
+ *  component so repeated Tailwind utilities collapse to the last-wins
+ *  value instead of fighting each other in the class attribute. */
+export function cn(...inputs: ClassValue[]): string {
+  return twMerge(clsx(inputs))
+}