feat(admin): build /admin/settings page for hub configuration

[YaronZaki]

Closes #1102
Closes #1103
Closes #1104
Closes #1105

Builds the admin-only Hub Settings page at /admin/settings, mirroring the existing admin pages (admin/payments, admin/announcements).

Page sections

1. Basic Info — Hub Name, Description, Contact Email, Contact Phone, Logo URL (with inline preview; a Cloudinary upload widget is planned as a follow-up).
2. Business Hours — per-day grid (Monday → Sunday) with an Open / Closed toggle and open / close time inputs.
3. Address — full address text field.
4. Tax & Currency — Tax Rate (numeric %, 0–100) and Currency (read-only NGN per [GF-FE-07] Build admin hub settings page /admin/settings #1102 acceptance criteria).
5. Timezone — dropdown of common African / global timezones (default Africa/Lagos).

A single Save settings button PATCHes /hub-settings via useUpdateHubSettings, shows a success toast on save, and shows the error message inline on failure.

New / changed files

• frontend/app/admin/settings/page.tsx (new) — the settings page itself.
• frontend/components/dashboard/DashboardSidebar.tsx — adds a Settings entry under adminItems so it's reachable from the admin sidebar (page inherits the existing /admin/* middleware guard, so only admins / super_admins can reach it).
• frontend/lib/react-query/hooks/admin/hub-settings/useGetHubSettings.ts (new) — wraps the existing GET /hub-settings endpoint.
• frontend/lib/react-query/hooks/admin/hub-settings/useUpdateHubSettings.ts (new) — wraps PATCH /hub-settings and invalidates the cache on success.

The hooks are tiny but were required for the page to function.

Acceptance criteria coverage

• frontend/app/admin/settings/page.tsx accessible via /admin/settings
• Page is admin / super_admin only (role check + middleware)
• Pre-populates from GET /hub-settings
• Save calls PATCH /hub-settings with a success / error toast
• Business Hours grid with toggle + time inputs for all 7 days
• Linked from the admin sidebar navigation
• Logo upload widget — see follow-up note: URL input + preview today, file upload via Cloudinary planned.

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