Fill frontend rule gaps and broaden v2 tokens

Add the frontend guides the v2 UI kit and compliance-portal need but
that the first rework left uncovered: forms, routing, client state, and
permission-gated UI.

forms.md documents a tiered approach on Base UI Field/Form -- native
constraints, then a validate function, then zod parsed in onSubmit, and
react-hook-form only for large or dynamic forms -- and drops the custom
useFormWithSchema wrapper. routing.md covers @probo/routes, navigation,
typed params, URL-as-state, redirects, auth/protected routes, and the
folded-in no-outlet-context rule. state-management.md gives a decision
order across Relay, URL, local state, context, and zustand.
permissions.md gates UI on the canUpdate/canDelete permission(action:)
fields without re-encoding authorization in the client.

Rename v2-colors.md to v2-tokens.md and add the typography, radius,
shadow, and native-spacing scales alongside color. Extend ui.md with
user feedback, empty-state, and accessibility sections; standardize
toasts on Base UI's Toast (Toast.useToastManager) and retire the legacy
useToast across ui.md, forms.md, error-handling.md, and relay.md. Add an
Intl formatting section to i18n.md and a non-Relay HTTP / file
upload-download section to ts-style.md. Update the AGENTS.md index and
the v2-color-scale cursor rule for the new and renamed guides.

Signed-off-by: Émile Ré <emile@probo.com>

.cursor/rules/v2-color-scale.mdc

@@ -16,4 +16,4 @@ Use the Radix 12-step numbered scale for all colors in v2 components: `sand`, `g
 - Do not mix v1 and v2 color classes in the same component
 - Respect step ranges: 1–2 backgrounds, 3–5 component bg, 6–8 borders, 9–10 solid bg, 11–12 text
 
-Full guide: `contrib/claude/v2-colors.md`
+Full guide: `contrib/claude/v2-tokens.md`