Wire browser-wallet build defines to the public RPC proxy

[AquiGorka]

What

Finishes the wallet RPC cutover started in #22. That PR replaced the hardcoded RPC URL with esbuild build-time defines (__SOROBAN_RPC_PROXY_{MAINNET,TESTNET,LOCAL}__), but those defines defaulted to an empty string and nothing sets them at package time:

• release.yml's deno task build-prod only sets POSTHOG_* — never the RPC proxy vars.
• The extension ships via manual Chrome Web Store / Mozilla Add-ons upload (no auto-publish), so there is no iac/CI step to inject them at build time.

So the next packaged build would ship an empty rpcUrl for mainnet/testnet → broken wallet.

Change

Bake the public, non-secret n-d-p read-only proxy URLs as the per-network defaults in src/build.ts, so a bare deno task build-prod is correct on its own. The RPC-Pro (Quasar) token stays server-side on n-d-p — only the public proxy URL ships. Operators/iac can still override any network via the SOROBAN_RPC_PROXY_* env vars.

network	baked default
mainnet	https://dashboard-api.moonlightprotocol.io/api/v1/public/rpc
testnet	https://dashboard-api-testnet.moonlightprotocol.io/api/v1/public/rpc
local	http://localhost:8000/soroban/rpc (no public proxy in local-dev; same host network.ts already fell back to)

Both live proxy hosts were verified read-only: getLatestLedger returns the correct network (mainnet seq ~62.9M, testnet seq ~3.0M), and sendTransaction is rejected by the allowlist.

Verification

Production build (deno task build-prod) inspected:

• Per-network rpcUrl resolves to the proxy URLs above (present in background.js + private-tracking.js).
• No RPC-Pro token, no lightsail host, no rpc-pro token in the shipped bundle. (lightsail appears only as a doc comment in .map files, which release.yml strips before packaging; quasar matches only a BIP39 mnemonic wordlist entry.)
• Remaining *.stellar.org / sorobanrpc.com strings are inert Stellar-SDK NetworkConfig library defaults, never used as the wallet's rpcUrl (the wallet builds via CustomNet({rpcUrl: <proxy>}) by design) — plus the intended public, token-free horizon/friendbot endpoints.

deno fmt --check, deno lint, and deno check all pass. (Repo has no test task/tests.)

Version bumped 0.1.2 → 0.1.3 (manifest.json).

Out of scope

The actual store publish (manual operator step), the n-d-p proxy itself (already deployed/verified), and the web-app frontends (already done in the RPC-proxy cutover).