feat: add /model /status /login slash commands and refactor dialog architecture

[lewis617]

Summary

• Replace gear button with /config, /plugin, /mcp slash commands opening three independent dialogs
• Add /model slash command — quick model and fast model switching dialog
• Add /status slash command — read-only status panel (version, session ID, workdir, model config, auth status)
• Add /login slash command — SSO authentication dialog (login/logout), moved out of ConfigDialog
• Update spec.md with new slash command documentation and screenshots
• Add e2e demo tests and webview tests for all new dialogs

Changes

New components:

• ModelDialog.tsx — model/fast model editor with pre-filled values
• StatusDialog.tsx — read-only status info (version, session, workdir, config, auth)
• LoginDialog.tsx — SSO auth dialog moved from ConfigDialog

Modified:

• ConfigDialog.tsx — removed SSO auth section (moved to LoginDialog)
• ChatApp.tsx — intercept /model /status /login, render new dialogs
• MessageInput.tsx — added new commands to localCommands
• messageHandler.ts — added getStatus/setModel handlers, new slash command definitions
• types/index.ts — extended activeDialog types and new dialog props

Tests:

• e2e/webview/model-status-login-commands.test.ts — 7 tests
• e2e/demo/model-dialog.demo.ts — 2 demo tests
• e2e/demo/status-dialog.demo.ts — 2 demo tests
• e2e/demo/login-dialog.demo.ts — 3 demo tests