feat: unify site theming via CSS custom properties (GH #91)

Replace all hardcoded brand color hex values with CSS custom properties
so BrandingContext drives both the customer portal and staff site.

- index.css: add derived accent/primary vars using color-mix()
  (--color-accent-hover, --color-accent-dark, --color-accent-light,
  --color-accent-lighter, --color-primary-dark); fix focus ring styles
  to use var(--color-primary) instead of hardcoded hex
- BrandingContext.tsx: also update <meta name="theme-color"> in sync
  with primaryColor so PWA theme-color tracks branding at runtime
- portal/sections: replace bg-[#8b7355], text-[#6b5a42], bg-[#f0ebe4],
  bg-[#faf5ef], hover:bg-[#7a6549] etc. with Tailwind v4 CSS var
  utilities (bg-(--color-accent), text-(--color-accent-dark), etc.)
- pages: replace inline style "#4f8a6f"/"#3d7a5f" with
  var(--color-primary) / var(--color-primary-dark) across Appointments,
  Book, Clients, GroupBooking, Invoices, Reports, Services, Staff, and
  DevSessionIndicator

Closes #91

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/pages/GroupBooking.tsx

@@ -287,7 +287,7 @@ function NewGroupBookingForm({
           <button
             type="submit"
             disabled={saving}
-            style={{ ...btnStyle, backgroundColor: "#4f8a6f", color: "#fff", borderColor: "#4f8a6f" }}
+            style={{ ...btnStyle, backgroundColor: "var(--color-primary)", color: "#fff", borderColor: "var(--color-primary)" }}
           >
             {saving ? "Booking…" : "Create Group Booking"}
           </button>
@@ -471,7 +471,7 @@ export function GroupBookingPage() {
         </select>
         <button
           onClick={() => setShowCreate(true)}
-          style={{ ...btnStyle, marginLeft: "auto", backgroundColor: "#4f8a6f", color: "#fff", borderColor: "#4f8a6f" }}
+          style={{ ...btnStyle, marginLeft: "auto", backgroundColor: "var(--color-primary)", color: "#fff", borderColor: "var(--color-primary)" }}
         >
           + New Group Booking
         </button>