Reconcile environment variables across code, .env.example, and the README

[priscaenoch]

Problem

The documented environment variables and the variables the code actually reads are out of sync:

• .env.example documents VITE_INDEXER_URL and VITE_NETWORK_PASSPHRASE, but neither is read anywhere in src/.
• The code reads VITE_API_URL (src/services/sandbox-api.ts) and VITE_SOROBAN_RPC_URL, but VITE_API_URL is not documented in .env.example.

This misleads anyone setting up the project (including the README, which currently describes VITE_INDEXER_URL as the URL the UI reads from — it does not).

What needs to be done

• Decide the canonical set of variables. Recommended: VITE_API_URL (indexer/API base, ties into the configurable-base issue), VITE_SOROBAN_RPC_URL, VITE_NETWORK_PASSPHRASE.
• Remove or rename the unused VITE_INDEXER_URL. If the intent was for the UI to read it, wire it; otherwise delete it.
• Update .env.example so every documented variable is actually consumed, and every consumed variable is documented.
• Update README.md Configuration table to match the final variable set.

Files

• .env.example
• README.md
• src/services/sandbox-api.ts
• any module that should read the network passphrase

Acceptance deliverables

• One-to-one correspondence between variables in .env.example, variables read in src/, and the README table.
• No dead or undocumented VITE_* variables remain.

Tests to pass

• A repo check (documented in the PR) confirming grep VITE_ src matches .env.example.
• Build passes with the finalized variables.