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>

app/globals.css

@@ -63,6 +63,10 @@
   --warning: 38 55% 50%;
   --warning-foreground: 0 0% 9%;
 
+  /* Attn: darker ochre for the one-sentence attention line (.attn / AttnLine),
+     derived from --warning but AA-safe as small text on the page background. */
+  --attn: 38 50% 34%;
+
   --warm-accent: 38 45% 52%;
 
   /* Chart colors */
@@ -127,6 +131,8 @@
   --warning: 38 50% 55%;
   --warning-foreground: 38 50% 90%;
 
+  --attn: 38 45% 62%;
+
   --warm-accent: 38 42% 58%;
 
   --chart-1: 155 22% 48%;
@@ -164,6 +170,7 @@
   --color-success-foreground: hsl(var(--success-foreground));
   --color-warning: hsl(var(--warning));
   --color-warning-foreground: hsl(var(--warning-foreground));
+  --color-attn: hsl(var(--attn));
   --color-warm-accent: hsl(var(--warm-accent));
   --radius-lg: var(--radius);
   --radius-md: calc(var(--radius) - 2px);