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/Staff.tsx

@@ -78,7 +78,7 @@ export function StaffPage() {
     <div style={{ fontFamily: "system-ui, sans-serif" }}>
       <div style={{ display: "flex", alignItems: "center", gap: "1rem", marginBottom: "1rem" }}>
         <h1 style={{ margin: 0 }}>Staff</h1>
-        <button onClick={openNew} style={{ ...btnStyle, backgroundColor: "#4f8a6f", color: "#fff", borderColor: "#4f8a6f", marginLeft: "auto" }}>
+        <button onClick={openNew} style={{ ...btnStyle, backgroundColor: "var(--color-primary)", color: "#fff", borderColor: "var(--color-primary)", marginLeft: "auto" }}>
           + Add Staff
         </button>
       </div>
@@ -145,7 +145,7 @@ export function StaffPage() {
               </div>
               {formError && <p style={{ color: "red", margin: "0.5rem 0 0" }}>{formError}</p>}
               <div style={{ display: "flex", gap: "0.5rem", marginTop: "1rem" }}>
-                <button type="submit" disabled={saving} style={{ ...btnStyle, backgroundColor: "#4f8a6f", color: "#fff", borderColor: "#4f8a6f" }}>
+                <button type="submit" disabled={saving} style={{ ...btnStyle, backgroundColor: "var(--color-primary)", color: "#fff", borderColor: "var(--color-primary)" }}>
                   {saving ? "Saving…" : editing ? "Save Changes" : "Add Staff"}
                 </button>
                 <button type="button" onClick={() => setShowForm(false)} style={btnStyle}>Cancel</button>