feat: add wallet network selection (testnet/mainnet/custom) (#889)

[adenueltech]

Closes #889

What changed

Before this PR, the network was fixed at build time through NEXT_PUBLIC_STELLAR_NETWORK and read in about 20 separate places. Users can now switch between Testnet, Mainnet and a Custom network in the app.

Network switching logic and stored preference: src/lib/networks.ts

• Presets for testnet and mainnet (passphrase, RPC, Horizon, contract ID, USDC, explorer), plus user-defined custom networks.
• The user's choice is saved in localStorage (stellarsplit_network). getActiveNetwork() returns the saved choice in the browser and the build default otherwise, including on the server.
• Switching reloads the page so every module-level constant and the splitClient are rebuilt for the new network. No stale RPC or contract client can leak across.

Endpoints correct per network

• config.ts, stellar.ts and freighter.ts now resolve RPC, Horizon, passphrase, contract ID and USDC from the active network. Existing exports (RPC_URL, NETWORK_PASSPHRASE, USDC_CONTRACT_ID, …) are unchanged, so callers keep working.
• Replaced the hard-coded NEXT_PUBLIC_STELLAR_NETWORK / NEXT_PUBLIC_RPC_URL ternaries in the client components and hooks (explorer links, Horizon polling, invoice stream, debugger, etc.). /payments previously always linked to the testnet explorer.
• New optional env vars NEXT_PUBLIC_{TESTNET,MAINNET}_{CONTRACT_ID,RPC_URL,HORIZON_URL,USDC_ADDRESS} (documented in .env.example). The un-prefixed vars apply only to the default network, so a testnet contract ID is never reused on mainnet.

Validate network compatibility

• A preset with no contract configured is shown as "Not configured for this deployment" and can't be selected.
• Custom networks are validated: https only (http allowed for localhost), passphrase required, contract addresses checked with StrKey. They must also pass a live getNetwork RPC call that reports the same passphrase before the switch is allowed.
• If Freighter is connected on a different network than the app, a warning banner says so (via getNetworkDetails).

Current network always visible, no accidental mainnet/testnet confusion

• New NetworkSelector badge in the navbar, now shown at every breakpoint. The old static badge was hidden below sm. It is colour-coded and opens the switch dialog.
• Switching to Mainnet requires ticking "I understand Mainnet uses real funds…".
• "Reset to default" clears the stored preference.
• NetworkStatus keeps the RPC health dot and now checks the active network's RPC.

Notes

• API routes (server) still use the env default network. The user's choice lives in the browser.

How I tested

• src/__tests__/networks.test.ts: presets, stored preference (including corrupt values), custom network validation and resolution, RPC passphrase check (match, mismatch, HTTP error, unreachable) and explorer URLs.
• npx tsc --noEmit: 0 errors. next lint on all changed files: clean.
• Full vitest run: 2141 passed, 5 failed. The 5 failures (ErrorBoundary, InvoiceExportButton, SubscriptionCard, brandTools, contractEvents) also fail or are flaky on unmodified main and are unrelated to this change.

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