Adopt whynot-design tokens (WP-0017 Phase 1)

Replace vergabe's blue brand-* palette with whynot's near-black/paper/yellow
visual language. Tokens vendored at static/src/vendor/whynot-design/ (synced
from commit 9419f16 via scripts/sync-whynot-design.sh / make sync-whynot-design).

main.css imports the vendored CSS first, exposes ink/paper/hi as Tailwind
@theme tokens (bg-paper, text-ink, border-line, etc.), and re-tones every
component class (.btn-*, .card, .field-row, .phase-*, .form-input, .table-*,
.sidebar-*). Border radii drop to whynot's 0-4px; .card loses its shadow.

Legacy text-brand-* / bg-brand-* / border-brand-* template references are
kept working via @theme aliases that map the old blue scale onto the whynot
ink ramp — Phase 1 is tokens-only, no template churn.

btn-danger keeps an off-spec red (#B22222) as a local --danger var until
upstream defines a canonical destructive color.

base.html body class swapped: bg-slate-50 → bg-paper-2 text-ink.

Phase 2 (component adoption) deferred until whynot-design ships Lit web
components + missing atoms (Card, Modal, Input, Table, Toast). See
wiki/DesignSystem.md and history/2026-05-23-whynot-design-cross-framework-analysis.md.

Verified: 8/8 e2e tests pass; dev server boots; static/dist/main.css contains
no #3b5bdb references. Visual pixel-level verification still pending Bernd's
browser walk.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help db dev css seed migrate test lint shell superuser collectstatic
+.PHONY: help db dev css seed migrate test lint shell superuser collectstatic sync-whynot-design
 
 .DEFAULT_GOAL := help
 
@@ -41,3 +41,6 @@ shell: ## Open a Django shell (shell_plus if available)
 
 collectstatic: ## Collect static files into staticfiles/ (production step)
 	uv run manage.py collectstatic --noinput
+
+sync-whynot-design: ## Re-vendor whynot-design CSS+tokens from the pinned ref
+	./scripts/sync-whynot-design.sh