feat(uat-batch-3): wave-1 primitives — DatePicker, DateTimePicker, FileInputButton, ColumnPicker hideAll

Builds the foundational primitives that subsequent waves depend on.
None of these introduce new deps — date-fns, react-day-picker, and
shadcn Calendar were already in the tree.

- `<DatePicker>` and `<DateTimePicker>` in src/components/ui — desktop
  popover wrapping the existing shadcn Calendar (caption-dropdown nav
  so reps can jump months/years for the SkipAheadBanner backfill UX),
  mobile native input via useIsMobile. Drop-in for `<Input type=date>`
  / `<Input type=datetime-local>`.
- `<FileInputButton>` in src/components/ui — styled Button + hidden
  input, replaces browser-default file picker UI. Most queued sweep
  sites already used the hidden-input + Button-trigger pattern; the
  primitive lands for any new caller plus consistent filename display
  + clear button.
- ColumnPicker `hideAll()` footer item — symmetric to existing
  `showAll()`, with the same visibility gate. Lands platform-wide via
  the shared component.
- Migrated highest-leverage call sites to the new primitives:
  * MilestoneAdvanceButton (backfill UX)
  * Reminder form (datetime-local → DateTimePicker)
  * Snooze dialog (datetime-local → DateTimePicker)
  * External-EOI upload dialog (date + file picker)
  * Payments section (received-on date)
- Remaining 15+ date-input call sites parked for a follow-up sweep —
  several use react-hook-form `register` patterns that need careful
  migration to the new controlled-value contract.

tsc clean. 1419/1419 vitest pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/reminders/reminder-form.tsx

@@ -3,6 +3,7 @@
 import { useState, useMemo } from 'react';
 import { useQuery } from '@tanstack/react-query';
 import { Button } from '@/components/ui/button';
+import { DateTimePicker } from '@/components/ui/date-time-picker';
 import { Input } from '@/components/ui/input';
 import { Label } from '@/components/ui/label';
 import { Textarea } from '@/components/ui/textarea';
@@ -304,12 +305,11 @@ function ReminderFormBody({
                   </button>
                 ))}
               </div>
-              <Input
+              <DateTimePicker
                 id="reminder-due"
-                type="datetime-local"
                 value={dueAt}
-                onChange={(e) => setDueAt(e.target.value)}
-                required
+                onChange={setDueAt}
+                placeholder="Pick a due date and time"
               />
             </div>
 

src/components/reminders/snooze-dialog.tsx

@@ -2,7 +2,7 @@
 
 import { useState } from 'react';
 import { Button } from '@/components/ui/button';
-import { Input } from '@/components/ui/input';
+import { DateTimePicker } from '@/components/ui/date-time-picker';
 import { Label } from '@/components/ui/label';
 import {
   Dialog,
@@ -91,13 +91,14 @@ export function SnoozeDialog({ open, onOpenChange, reminderId, onSuccess }: Snoo
           <div className="space-y-2 pt-2">
             <Label htmlFor="custom-snooze">Custom date & time</Label>
             <div className="flex gap-2">
-              <Input
-                id="custom-snooze"
-                type="datetime-local"
-                value={customDate}
-                onChange={(e) => setCustomDate(e.target.value)}
-                className="flex-1"
-              />
+              <div className="flex-1">
+                <DateTimePicker
+                  id="custom-snooze"
+                  value={customDate}
+                  onChange={setCustomDate}
+                  placeholder="Pick a date and time"
+                />
+              </div>
               <Button
                 disabled={loading || !customDate}
                 onClick={() => handleSnooze(new Date(customDate).toISOString())}