fix(layout): mobile UX cleanup + interest-stage legend popover

Mobile UX:
- Hide ColumnPicker on `< sm` viewports (cards, no columns to toggle).
- Hide kanban toggle in interest list on mobile and snap viewMode back
  to 'table' if the persisted choice was 'board'.
- Drop dead "Inbox" link from the More-sheet (email/IMAP feature is
  deferred per sidebar.tsx note).
- Repoint Notifications nav from `/notifications` (no page.tsx — 404)
  to `/notifications/preferences` and re-label as "Notification
  preferences" (the bell stays the surface for actual notifications).
- Hide Website Analytics on both desktop sidebar and mobile More-sheet
  when Umami isn't configured for the port (`useUmamiActive()`).

Interests:
- New `<StageLegend>` popover button in the filter row decodes the
  card stripe colours to pipeline stage names, kept in sync with
  `STAGE_DOT` automatically.

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

src/components/shared/column-picker.tsx

@@ -68,9 +68,12 @@ export function ColumnPicker({
   return (
     <DropdownMenu>
       <DropdownMenuTrigger asChild>
-        <Button variant="outline" size="sm" className="gap-1.5 h-8">
+        {/* Hide entirely on mobile — small viewports render the list as
+            cards (no columns to pick). The desktop table view shows
+            this trigger from the `sm:` breakpoint up. */}
+        <Button variant="outline" size="sm" className="hidden sm:inline-flex gap-1.5 h-8">
           <Columns3 className="h-3.5 w-3.5" />
-          <span className="hidden sm:inline">Columns</span>
+          <span>Columns</span>
         </Button>
       </DropdownMenuTrigger>
       <DropdownMenuContent align="end" className="w-56">