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/tailwind.config.js

@@ -33,6 +33,8 @@ export default {
         'fade-in': 'fadeIn 0.2s ease-in-out',
         'slide-up': 'slideUp 0.3s ease-out',
         'shimmer': 'shimmer 2s infinite linear',
+        'accordion-down': 'accordion-down 0.2s ease-out',
+        'accordion-up': 'accordion-up 0.2s ease-out',
       },
       keyframes: {
         fadeIn: {
@@ -47,8 +49,17 @@ export default {
           '0%': { backgroundPosition: '-200% 0' },
           '100%': { backgroundPosition: '200% 0' },
         },
+        // Used by shadcn Accordion / Collapsible.
+        'accordion-down': {
+          from: { height: '0' },
+          to: { height: 'var(--radix-accordion-content-height)' },
+        },
+        'accordion-up': {
+          from: { height: 'var(--radix-accordion-content-height)' },
+          to: { height: '0' },
+        },
       },
     },
   },
-  plugins: [],
+  plugins: [require('tailwindcss-animate')],
 }