test: NetworkSwitcher/ButtonGroup/SorobanInvokeButton coverage (issue #346)

[BernardOnuh]

Summary

Adds the missing test coverage requested in #346. Most scenarios named in this issue (Alt+N shortcut, mismatch badge, NetworkSwitcher localStorage persistence, ButtonGroup icons and href-disabled behavior) were already covered by existing tests, so this PR targets the genuine gaps only.

Changes

• SorobanInvokeButton (SorobanInvokeButton.test.tsx)
  • Loading label shows 'Invoking {method}...' while an invocation is in flight, and 'Invoking...' when a custom label is provided.
  • tooltip prop renders as the button title when connected; falls back to 'Connect wallet to invoke' when disconnected.
  • Result container caps its height via maxResultHeight (default 200px and custom values).
• ButtonGroup (Button.test.tsx)
  • Hardened the border-radius tests to assert the [&>:not(:first-child)]:rounded--none child selectors that collapse the shared radius on sibling buttons.

Test output

``'
✓ src/components/ui/Button.test.tsx (30 tests)
✓ src/components/SorobanInvokeButton.test.tsx (16 tests)

Test Files 2 passed (2)
Tests 46 passed (46)
``'

Note: a full npm test reports pre-existing failures in unrelated files (NFTGallery, ActivityTimeline, RewardsPanel, PortfolioRebalancer, etc.) caused by a @hugeicons/react incompatibility. These fail identically on the base commit and are out of scope for this issue.

Closes #346

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