refactor(cards): Phase A + C — adopt @mana/shared-theme + per-app accent

Phase A — Cards joins the unified theme system:
- Drop placeholder --color-cards-* palette; app.css imports
  @mana/shared-tailwind/themes.css + sources.css.
- Remove hardcoded class="dark" from app.html; body uses
  bg-background text-foreground.
- New $lib/stores/theme.ts: createThemeStore({ appId: 'cards' }).
  ThemeToggle from @mana/shared-theme-ui in the header next to
  the streak chip.
- Sweep all neutral / red / emerald / amber / indigo utilities in
  apps/cards/apps/web/src to semantic tokens (560 substitutions
  across 19 files): bg-neutral-900 → bg-card, text-neutral-400 →
  text-muted-foreground, bg-red-500 → bg-error, etc. Domain
  literals kept (FSRS grade colors red/orange/green/blue, GitHub-
  violet PR-merged badge, marketplace-amber Buy button, admin-
  inbox category palette).
- Cards added to validate-theme-utilities scope so future drift
  fails CI.

Phase C — per-app accent token:
- New --color-app-accent in shared-tailwind/themes.css. Theme-
  agnostic (registered in validate-theme-parity's THEME_AGNOSTIC
  regex), so it stays the same across light/dark/lume/etc. Defaults
  to Mana indigo at :root.
- Cards layout writes 258 90% 66% (= #8b5cf6 violet, from
  MANA_APPS.cards.color) onto documentElement at boot via
  applyCardsAccent(). All Cards CTAs (Lernen, Abonnieren, Senden,
  links inside cloze cards) flow through bg-app-accent /
  text-app-accent now.

Net effect: Cards gets light/dark + 4 palette variants + a11y
toggles for free, and any future app can drop in by setting its
own --color-app-accent without touching shared-tailwind.

packages/shared-tailwind/src/themes.css

@@ -98,6 +98,13 @@
 	--color-card-foreground: hsl(var(--color-card-foreground) / <alpha-value>);
 	--color-accent: hsl(var(--color-accent) / <alpha-value>);
 	--color-accent-foreground: hsl(var(--color-accent-foreground) / <alpha-value>);
+
+	/* Per-app brand accent — set by the host app's layout via
+	   `style="--color-app-accent: <H S% L%>"` on documentElement.
+	   Theme-agnostic: stays the same across light/dark/lume/etc. so
+	   the app's identity reads consistently. Defaults to Mana brand
+	   below in :root if no host overrides. */
+	--color-app-accent: hsl(var(--color-app-accent) / <alpha-value>);
 }
 
 @theme {
@@ -170,6 +177,13 @@
 	--color-branch-social: 38 92% 50%;       /* amber — warmth, relationships */
 	--color-branch-practical: 173 80% 40%;   /* teal — craft, tools */
 	--color-branch-mindset: 142 71% 45%;     /* green — calm, growth */
+
+	/* Per-app brand accent default — Mana indigo. Host apps override
+	   at runtime by writing `--color-app-accent: <H S% L%>` onto
+	   documentElement, typically from MANA_APPS[<id>].color via the
+	   hexToHsl() helper in @mana/shared-theme/utils. Theme-agnostic:
+	   not redefined in .dark / [data-theme="…"] blocks. */
+	--color-app-accent: 239 84% 67%;
 }
 
 /* ===== Default Theme (Lume Light) ===== */