Fix mobile faucet link icons

[KevinMehrabi]

What changed

• replace platform-dependent external-link characters with monochrome Material icons
• apply the same treatment to all sTEZ and testnet faucet links
• add regression coverage for the rendered icons

Why

iOS Safari can render the Unicode northeast arrow as a colored emoji, making the faucet buttons look inconsistent with the TEZEX interface.

Impact

External faucet links now use a fixed-size vector icon that inherits the button color consistently across desktop and mobile browsers.

Validation

• npm run typecheck
• Home and sTEZ test suites: 6 tests passed
• npm run build
• mobile visual verification at 390 × 844
• no browser console errors