test: raise coverage to 95% across all metrics

[WiktorStarczewski]

Summary

• Raise jest coverageThreshold to 95% for statements, branches, functions, and lines
• Rename CI coverage job to "Coverage Check (95% minimum)" in pr.yml
• Add 562 new tests across 30 new test files and extend 32 existing test files
• Fix jest.setup.js crypto polyfill (defineProperty instead of Object.assign) and add TextEncoder/TextDecoder for jsdom
• Remove dead code in dapp.ts (unreachable branch guards, redundant ternaries)
• Add /* c8 ignore */ for fundamentally untestable branches (SSR guards, build-time constants, re-entrancy guards)

Coverage before → after

Metric	Before	After
Statements	73.74%	95.74%
Branches	86.16%	95.34%
Functions	66.57%	95.69%
Lines	73.74%	95.74%
Tests	1,079	1,641

Test plan

• All 1,641 tests pass (yarn test)
• yarn test:coverage passes at the 95% threshold
• No source logic changed (only test files, jest config, CI workflow, and dead-code removal)