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/shared/column-picker.tsx

@@ -59,11 +59,18 @@ export function ColumnPicker({
     onChange([]);
   }
 
+  function hideAll() {
+    onChange(columns.filter((c) => !c.alwaysVisible).map((c) => c.id));
+  }
+
   // The "All visible" affordance is only useful when something is
   // hidden — a no-op button is noise.
   const canShowAll = hidden.some((id) =>
     columns.some((col) => col.id === id && !col.alwaysVisible),
   );
+  // Mirror: "Hide all" is only useful when at least one toggleable
+  // column is currently visible.
+  const canHideAll = columns.some((col) => !col.alwaysVisible && !hiddenSet.has(col.id));
 
   return (
     <DropdownMenu>
@@ -113,13 +120,16 @@ export function ColumnPicker({
             </DropdownMenuItem>
           );
         })}
+        {(canShowAll || canHideAll) && <DropdownMenuSeparator />}
         {canShowAll && (
-          <>
-            <DropdownMenuSeparator />
-            <DropdownMenuItem onClick={showAll} className="text-xs text-muted-foreground">
-              Show all columns
-            </DropdownMenuItem>
-          </>
+          <DropdownMenuItem onClick={showAll} className="text-xs text-muted-foreground">
+            Show all columns
+          </DropdownMenuItem>
+        )}
+        {canHideAll && (
+          <DropdownMenuItem onClick={hideAll} className="text-xs text-muted-foreground">
+            Hide all columns
+          </DropdownMenuItem>
         )}
         {onSaveView && (
           <>