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/duplicates/DuplicatesView.tsx

@@ -2,6 +2,7 @@ import { useMemo, useState, useEffect, useCallback } from 'react'
 import { Copy, Layers, Sparkles, Trash2, Loader2, Info, Crown } from 'lucide-react'
 import clsx from 'clsx'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
+import { Button } from '@/components/ui/button'
 import {
   useDuplicateGroupsQuery,
   DUPLICATE_GROUPS_QUERY_KEY,
@@ -269,7 +270,9 @@ function DuplicateGroupSection({
             )}
           </span>
         </div>
-        <button
+        <Button
+          variant="outline"
+          size="sm"
           onClick={() => {
             const discardIds = group.members
               .filter((m) => m.id !== best.id)
@@ -278,16 +281,12 @@ function DuplicateGroupSection({
             onKeepBest(discardIds)
           }}
           disabled={isPending}
-          className={clsx(
-            'flex items-center gap-1.5 rounded border border-border px-2 py-1 text-xs font-medium transition-colors',
-            'hover:border-reject/50 hover:bg-reject/10 hover:text-reject',
-            isPending && 'cursor-not-allowed opacity-50'
-          )}
+          className="hover:border-reject/50 hover:bg-reject/10 hover:text-reject"
           title="Keep the highest-resolution copy and discard the rest"
         >
-          <Trash2 className="h-3 w-3" />
+          <Trash2 className="mr-1.5 h-3 w-3" />
           Keep best, discard {discardCount}
-        </button>
+        </Button>
       </header>
 
       <div