Stabilize CI and cover number converter

[vuon9]

Summary

• add focused tests for internal/numberconverter conversions, views, invalid input, file-size, and duration formatting
• restore Wails v3 alpha.95 because the merged alpha.8.3 dependency update removes APIs this app still uses and breaks go build .
• add a Vitest localStorage fallback for current Bun/jsdom runs where window.localStorage is absent

Verification

• go test ./internal/...
• go test ./internal/... -coverprofile=/tmp/devtoolbox-internal-cover.out (internal/numberconverter: 98.1%)
• go build .
• cd frontend && bun run format:check
• cd frontend && bun run test (31 tests)
• cd frontend && bun run build

Notes

PR #93 remains blocked: @vitejs/plugin-react@6.0.1 requires Vite ^8.0.0, while this repo is on Vite 5.4.21. That should be deferred or handled as a Vite major upgrade, not merged as a standalone Dependabot bump.

[github-actions]

Package	Line Rate	Health
devtoolbox/internal/barcode	93%	✔
devtoolbox/internal/codeformatter	80%	➖
devtoolbox/internal/converter	65%	➖
devtoolbox/internal/datagenerator	81%	✔
devtoolbox/internal/datetimeconverter	75%	➖
devtoolbox/internal/jwt	47%	❌
devtoolbox/internal/numberconverter	98%	✔
devtoolbox/internal/settings	0%	❌
devtoolbox/internal/themes	0%	❌
Summary	70% (3728 / 5294)	➖

Minimum allowed line rate is 60%