fix(env): make kongSwapUrl optional

[yhabib]

Motivation

When the canister is installed in some environments, the app crashes because the kongSwapUrl environment variable is missing from the HTML meta tag. KongSwap is only a fallback ticker provider (ICP Swap is primary), so it shouldn't be mandatory.

Changes

• Made kongSwapUrl optional in the EnvironmentVars type and removed it from mandatory env var keys.
• Added an early check in queryKongSwapTickers to throw a clear error when the URL is not configured, which is caught by loadTickers gracefully.
• Updated the env-vars test to assert that Kong Swap URL is not mandatory.

Tests

• ...

Todos

• Accessibility (a11y) – any impact?
• Changelog – is it needed?

[0xFFrancesco]

The same logic should apply to IcpSwap then?

IIUC it was fixed with DFX 0.31, so we could rather say to use the new DFX version.

wdyt?