feat(ui): shared migration primitives (UI migration PR 3) (#1122)

* feat(ui): shared migration primitives (UI migration PR 3)

The component kit every page migration (PR 4-8) builds on:

- ContextPicker: the one-per-page chip-dropdown context scope (convention
  8), right-aligned popover with checks and muted annotations
- FyPicker: fiscal-year picker on ContextPicker with the same controlled
  API and per-company localStorage key as FiscalYearSelector, which it
  replaces page by page from PR 4
- SplitButton: primary + caret menu, last-used mode persisted per user
  via ui_state.create_mode (lib/ui-state/client, unit-tested); nav
  persistence refactored onto the same helper
- ConfirmDialog: centered min-460px confirm-up-front dialog (convention
  10) with pending state on an awaitable onConfirm
- HelpPopover: 17px "?" after the H1 opening an anchored popover
  (convention 7); PageHeader gets a `help` slot
- AttnLine: the one-ochre-sentence attention pattern (convention 6) with
  optional inline action; new AA-safe --attn token pair
- RowStatus: chips-mark-exceptions helper (convention 5)
- SlideOver: right review panel, 480px, 18px inset, rounded, veil + Esc
  (convention 13), with header kicker / body / footer slots
- Stagger: .stagger-enter applied to the five target pages' list
  containers (bookkeeping, transactions, pending, invoices,
  supplier-invoices); structural loading.tsx added for supplier-invoices,
  customers, kpi, pending, deadlines

No page adopts the new pickers/dialogs yet: that is PR 4-8, one page per
PR against this kit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(ui): FyPicker chip must not double the Rakenskapsar label

Real fiscal periods are often named "Rakenskapsar 2026" already; only
prefix the label when the period name lacks it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

messages/en.json

@@ -63,7 +63,8 @@
       "sent": "Sent",
       "matched": "Matched",
       "unmatched": "Unmatched"
-    }
+    },
+    "more_options": "More options"
   },
   "nav": {
     "dashboard": "Overview",

messages/sv.json

@@ -63,7 +63,8 @@
       "sent": "Skickad",
       "matched": "Matchad",
       "unmatched": "Omatchad"
-    }
+    },
+    "more_options": "Fler alternativ"
   },
   "nav": {
     "dashboard": "Översikt",