chore: route both testnet and mainnet via gh secrets

[AquiGorka]

Summary

• Both deploy jobs now read stellarNetwork and rpcUrl from gh secrets instead of hardcoded URLs:
  • testnet: STELLAR_NETWORK + RPC_URL
  • mainnet: MAINNET_STELLAR_NETWORK + MAINNET_RPC_URL
• Removes the gateway.fm hardcoding from the mainnet job and the soroban-testnet hardcoding from the testnet job.
• deno.json patch bump 0.2.5 -> 0.2.6.

Repo gh secrets set in the same change:

• STELLAR_NETWORK = testnet
• RPC_URL = https://soroban-testnet.stellar.org
• MAINNET_STELLAR_NETWORK = mainnet
• MAINNET_RPC_URL = https://rpc-pro.lightsail.network/ (Quasar Pro)

Note for PM thread

Phase 0 finding #3 said network-dashboard derives Stellar config at runtime from council-platform and makes no direct RPC calls. Verified during D4: dashboard DOES make direct RPC calls — src/lib/stellar.ts:139 (new s.rpc.Server(RPC_URL)) is used by getChannelSupply and other contract reads. ClickUp 86c9n8cby (proxy refactor) scope should be extended to include network-dashboard.

The Quasar token IS embedded in the dashboard's mainnet public/config.js as the accepted exposure for the silent release.

Part of the silent mainnet flip (ClickUp 86c9c86b6).

Test plan

• deno fmt clean
• deno lint clean
• deno task test — 28 passed
• Tag-triggered mainnet deploy generates a public/config.js with stellarNetwork: "mainnet" and Quasar rpcUrl
• Tag-triggered testnet deploy generates a public/config.js with stellarNetwork: "testnet" and soroban-testnet rpcUrl
• UI smoke on https://dashboard.moonlightprotocol.io