feat(ui): shared confirmation, toasts, error, skeleton, and Back primitives

Stage 1's later screens and modules need the interactions DESIGN.md
fixes once for the product (ADR 0059). @ekklesia/ui gains them,
additively:

- ConfirmationDialog: simple, destructive, or typed-phrase
  confirmation, with severity chosen by the caller. Destructive is
  marked by symbol and frame, not colour alone, and starts on Cancel.
  A required phrase must match exactly (forgiving only surrounding
  whitespace and NFC). While busy, Cancel and Escape are disabled.
- ToastProvider / useToasts: a top-center toast system per React tree,
  never a package singleton (ADR 0040). Live regions are mounted
  before any toast arrives; errors are assertive and persist until
  dismissed; routine toasts last at least three seconds and longer for
  longer text. Timers pause on hover and focus; duplicates refresh;
  errors are never displaced; every timer is cleared on unmount.
- ErrorState: recovery actions, plus collapsed technical details and a
  reference, rendered as bounded inert text.
- Skeleton / SkeletonGroup: an aria-busy region labelled once, with
  every animation stopped under prefers-reduced-motion.
- PageHeader `back` (link or button) above the page's one h1.
- Dialog: the focus trap ADR 0039 deferred, plus focus restoration and
  an accessible description. Escape is now handled on the dialog
  itself, so a drawer it was opened from stays open.

The shell validates Back targets (parseReturnTarget, dashboardBack):
only a canonical Dashboard page on this origin is accepted, otherwise
Back goes to the page's parent. It never uses history.back().

Biome's scope-aware suspicious/noAlert is now an error in the root
biome.json. tests/native-dialogs.test.ts proves it catches real
dialogs and not lookalikes, and scans for the forms the rule misses.
It supersedes the shell-only regex test.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture/platform-architecture.md

@@ -592,7 +592,11 @@ registry that constructs is one that can be emitted as CSS without escaping. Tha
 exactly what makes a future theme editor implementable rather than aspirational: the values it would
 edit are already labelled, described, categorised, and enumerable. `@ekklesia/ui` is the small set of
 elements a real module dashboard cannot be built without, every value a token reference, with real
-`:focus-visible` treatment and status that never depends on colour alone. `pnpm boundaries` opens
+`:focus-visible` treatment and status that never depends on colour alone. It now also carries the
+shared interactions — a focus-trapping `Dialog` and `ConfirmationDialog`, a per-tree toast system,
+`ErrorState` with collapsed technical detail, skeletons, and `PageHeader`'s Back — and the lint
+refuses browser-native dialogs repository-wide
+([ADR 0059](decisions/0059-shared-interaction-primitives.md)). `pnpm boundaries` opens
 exactly two shared roles to modules **by name**, and pins both design packages beneath everything
 else. The Dashboard and display install the defaults at their document root; custom properties
 inherit into every module artifact, so a module reads `var(--ekklesia-…)` with nothing to import at