feat(network): mainnet RPC + settings switcher (Tier 3.1)

[epicexcelsior]

Summary

• Adds Settings → Network row that opens a bottom-sheet for switching between mainnet beta / devnet / custom RPC. Preference persists to AsyncStorage and takes priority over the build-time EXPO_PUBLIC_SOLANA_RPC env default.
• Mainnet selection is gated behind a one-time confirmation alert ("real funds, not audited") with the acknowledgement persisted in PrefKeys.MAINNET_ACK.
• Recovery-key modal surfaces an extra mainnet reminder when the runtime cluster is mainnet beta (T-WALLET-* threat-model family).
• Explorer URLs now honour the runtime cluster — mainnet drops the ?cluster= query (the explorer default), devnet/testnet/custom map correctly, fixing the previous always-?cluster=devnet hardcode.

Files

• src/infrastructure/network/preference.ts (new): pref types, layered URL resolution (pref → env → public devnet), cached load + change emitter, mainnet/devnet/testnet defaults, URL validator.
• src/infrastructure/network/connection.ts: adds getEffectiveRpcUrl() / getActiveSolanaConnection(); singleton rebuilt on every pref change via subscriber. Legacy solanaConnection export kept for back-compat.
• context/NetworkModeContext.tsx: exposes pref / cluster / rpcUrl / setPref; rebuilds DirectRpcAdapter when pref.url changes so live consumers (balance / send) pick up the new endpoint without restart.
• components/settings/NetworkSwitcherSheet.tsx (new): radio selection + validated custom URL input, mainnet confirmation gate, active-URL preview.
• components/settings/ExportWalletModal.tsx: mainnet-only banner above the recovery-key warning.
• screens/SettingsScreen.tsx: new "solana cluster · {current}" row in the network section.
• src/services/explorer.ts: cluster-aware URL builder.
• src/storage/index.ts: NETWORK_PREF + MAINNET_ACK key constants.

Singleton rebuild — full / restart-required / not-attempted

Full. The solanaConnection export is reassigned in-place on every pref change (subscribed module-side), and useNetworkMode()'s DirectRpcAdapter re-mounts on pref.url change, so the live UI paths (balance refresh, send transaction blockhash / submit) follow the new endpoint without restart. A restart hint remains in the sheet UI as belt-and-suspenders for any caller that captured the connection at module-load and never re-reads it.

Coordination with anonmesh#52 (NetworkBanner)

Banner was NOT extended in this PR — it doesn't exist on this branch yet. PR anonmesh#52 ships the env-only banner; a follow-up after both merge will extend it to read the runtime pref via useNetworkMode().cluster. Merge order: anonmesh#52 first, then this PR.

Safety rails respected

• MeshRpcAdapter routing untouched (fix(send): off-grid resilience — timeouts, render-time gate, adapter-race log anonmesh/mobile_app#53's territory)
• sendTransaction send flow untouched (fix(send): off-grid resilience — timeouts, render-time gate, adapter-race log anonmesh/mobile_app#53's territory)
• All changes additive — no existing signatures modified
• No dep changes, no LOCAL_NOTES/ edits

Test plan

• Open Settings → tap "solana cluster · devnet" row → sheet opens with devnet radio selected
• Switch to mainnet → confirmation alert fires ("Real funds...") → tap Continue → row updates to "mainnet beta"
• Switch back to mainnet a second time → alert does NOT re-fire (acknowledgement persisted)
• Pick custom → paste valid https URL → save button enables → save → row reflects custom
• Pick custom → paste "not a url" → save button disabled, inline error shown
• After mainnet switch → open Settings → Reveal recovery key → mainnet reminder banner appears above the standard recovery warning
• On mainnet, complete a send → transaction explorer link opens explorer.solana.com (no ?cluster=devnet)
• Force-quit + relaunch → preference persists across app launch

Generated with Claude Code.

[epicexcelsior]

Re-routing to anonmesh/mobile_app — wrong repo target by mistake.