docs: full product tour with real human-agent workflow screenshots

[veithly]

What changed

Rewrote the English, Simplified Chinese and Japanese READMEs as a full product introduction rather than a 1.1 feature list. The source-backed tour covers shared sessions and activity, approvals, local coding-agent launch/resume modes, Git diff review, ordinary completion, opt-in AI prediction and its data flow, quick commands/snippets, local and remote files, split/detached layouts, custom CSS and recovery, plugins, forwarding, recordings and optional sync.

Added five actual 1440x900 UI screenshots with synthetic Northstar data: collaboration/history, approval, agent launcher, agent changes review and connection cards. This is the real React application behind the official in-memory Tauri mock bridge, not a separately drawn UI. No live servers, credentials, model calls or service restarts. The fixture is a dedicated development-only loopback entry and is absent from production JS. Reproduction instructions include the source paths behind the documented capabilities.

Validation

• Release metadata, translated README links and 12 plugin references: pass
• Dedicated demo TypeScript check: pass
• Frontend: 52 files / 249 tests pass
• Production build and cargo check --workspace --locked: pass
• Five PNG headers/dimensions and identical gallery references in all three READMEs: pass
• Production code, dependencies and version unchanged

Boundaries

These screenshots demonstrate the UI with synthetic responses, not real agent execution or SSH transport verification. An attempted additional file/plugin capture was blocked by the platform and was not repeated. Pixel-level native reinspection was limited by the image response budget and local accessibility permissions; DOM/content and error-state checks were completed. Existing build/test warnings remain. No installed application or user session was changed. This PR targets dev; it does not alter the 1.1.0 tag or publish a release.