feat: add first-run onboarding and prepare v1.9.4

[weidonglang]

Scope

• add a four-step bilingual first-run guide after the safety notice
• persist completion in settings, migrate legacy settings safely, and allow manual reopen from Settings
• include the already merged v1.9.4 Cleanup task views and one-flow safe port release in release metadata
• extend feature, selector, localization, and visual acceptance for onboarding
• prepare exact v1.9.4 manifests and release record

Verification

• Rust: 231 tests passed; fmt and Clippy with -D warnings passed
• frontend build: 114 modules passed
• frontend acceptance: 293 selectors passed
• Edge visual acceptance: 34/34 passed
• combined feature acceptance: 303 cases, 256 passed, 0 failed, 41 safe skips, 6 manual/policy
• backend/UI drift, Tauri command, data/action/quality/i18n, safety, hygiene, migration, and release consistency checks passed

Assets built from product commit 9e1391b

• NSIS: 2,962,510 bytes, SHA256 ad0b569ace83dc7b224fe841b97baa3fd6e15215c384e92a1d98f33493f644c1
• MSI: 4,952,064 bytes, SHA256 ec492b921a004324c5958d01231b177dba82ecf73e418df43e02a7a53ab40f62

Boundaries

• no high-risk system mutation was repeated for this onboarding-only source change
• installers remain unsigned, as disclosed in the release record
• Runtime lifecycle follow-up：provider-managed install/uninstall 与生态扩展 #130 and 应用内验收中心与安全后端验收 API（后续） #133 retain their remaining scope