Nuke buy providers + flip node defaults from Cake to community

Buy providers (Robinhood Connect, DFX, Onramper, MoonPay, Kryptonim, Meld):
- lib/entities/provider_types.dart: getAvailable{Buy,Sell}ProviderTypes()
  both return [] now. Each provider requires a formal partner contract
  with revenue-share — no free integrations exist. Re-add to the list
  once Such Software has registered the partner relationship per provider.
  Buy/Sell UI surfaces (cards_view.dart's onramp button -> Routes.buySellPage)
  still render but the provider list is empty, so the screen shows nothing.
- lib/view_model/support_view_model.dart: removed the third-party support
  LinkListItem entries for the dead buy providers. Kept the swap-provider
  support links (ChangeNow, SideShift, SimpleSwap, Exolix, SwapTrade,
  Trocador) — Trocador is still active and the others are useful for
  legacy users who created swaps before the trim.

Default node flip (per user request — 'someone else top choice, Cake nodes
stay in the list of others'):
- xmr-node.cakewallet.com:18081  -> node.sethforprivacy.com:443
- electrum.cakewallet.com:50002  -> electrum.blockstream.info:50002
- ltc-electrum.cakewallet.com:50002 -> electrum-ltc.bysh.me:50002
- btc-electrum.cakewallet.com:50002 -> electrum.blockstream.info:50002
Cake's nodes remain in the augmented lists (see asset/*_node_list.yml).
Silent-payments Electrs (electrs.cakewallet.com) + zcash default URI
unchanged — no good free alternative for silent payments, and zcash is
removed anyway.

lib/view_model/support_view_model.dart

@@ -63,37 +63,7 @@ abstract class SupportViewModelBase with Store {
               icon: 'assets/images/trocador.png',
               linkTitle: 'mail@trocador.app',
               link: 'mailto:mail@trocador.app'),
-          LinkListItem(
-              title: 'Onramper',
-              icon: 'assets/images/onramper_dark.png',
-              lightIcon: 'assets/images/onramper_light.png',
-              linkTitle: 'View exchanges',
-              link: 'https://onramper.com/'),
-          LinkListItem(
-              title: 'DFX',
-              icon: 'assets/images/dfx_dark.png',
-              lightIcon: 'assets/images/dfx_light.png',
-              linkTitle: 'support@dfx.swiss',
-              link: 'mailto:support@dfx.swiss'),
           if (!isMoneroOnly) ...[
-            LinkListItem(
-                title: 'MoonPay',
-                icon: 'assets/images/moonpay.png',
-                linkTitle: S.current.submit_request,
-                link: 'https://support.moonpay.com/hc/en-gb/requests/new'),
-            LinkListItem(
-                title: 'Robinhood Connect',
-                icon: 'assets/images/robinhood_dark.png',
-                lightIcon: 'assets/images/robinhood_light.png',
-                linkTitle: S.current.submit_request,
-                link: 'https://robinhood.com/contact')
-          ],
-          LinkListItem(
-              title: 'Kryptonim',
-              icon: 'assets/images/kryptonim_dark.png',
-              lightIcon: 'assets/images/kryptonim_light.png',
-              linkTitle: 'contact@support.kryptonim.com',
-              link: 'mailto:contact@support.kryptonim.com'),
         ];
 
   final docsUrl = 'https://github.com/Such-Software/hash-wallet';