Simplify connection setup with JSON profiles

[suzdalenko-dev]

What changed

• store each Simple DB connection in its own readable JSON file
• automatically migrate existing 0.1.1 connection profiles
• keep passwords in VS Code SecretStorage and reject password fields in JSON
• simplify Create Connection to engine + name + optional secure password, then open the JSON
• add Open Connection JSON, Set Password, and Open Connections Folder actions
• add a native Simple DB editor context submenu with six primary actions
• make the empty engine row directly clickable to create a connection
• document the new workflow entirely in English

Why

The previous connection flow stored profiles only in VS Code globalState and required many sequential input dialogs. After installation it was not obvious where connection settings lived or how to edit them. The new flow makes connection configuration visible, editable in one place, and easier to discover while preserving secure password storage.

Validation

• npm run check — ESLint clean, 54/54 tests pass
• npx @vscode/vsce package --out /tmp/simple-db-simplified.vsix — VSIX packages successfully
• npm audit --omit=dev — 0 vulnerabilities
• remote branch comparison — 1 commit ahead of main, 0 behind, exactly 8 intended files changed