feat(external-sources): improve application connection experience

[limityan]

Summary

• reshape External Sources Settings into an application-first overview with result-first detail and advanced controls
• persist backend-owned external application awareness and expose a quiet Settings navigation dot without using localStorage
• align GUI slash-command status with the existing TUI semantics for restricted and unresolved-source commands
• add guarded connection confirmation, application summaries, source awareness, and focused tests
• keep existing V1 capability controls and host safety fencing intact while preparing the UI boundary for the later application review model

Product behavior

• the Settings hint means only that a newly discovered external application has not been shown to the user; content changes do not re-trigger it
• awareness is persisted in the external-source preferences file, scoped by execution domain, and does not change policy preference revision
• connection confirmation explains that low-risk commands may become available automatically while executable tools, agents, and MCP servers remain subject to confirmation
• system diagnostics are not counted as user review items

Scope

This PR is a staged frontend-first implementation. It improves the current V1 experience and introduces the UI/awareness boundaries for the planned application-level model; it does not claim that the full V2 batch review protocol is implemented.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/utils/externalPromptCommands.test.ts src/flow_chat/components/ChatInput src/infrastructure/api/service-api/ExternalSourcesAPI.test.ts src/infrastructure/config/components/ExternalSourcesConfig.test.tsx src/infrastructure/config/components/external-sources src/app/scenes/settings
• cargo test -p bitfun-core --features external-sources --lib external_sources::tests::acknowledg
• cargo test -p bitfun-desktop --lib remote_workspace_policy
• pnpm run check:repo-hygiene
• pnpm run check:core-boundaries
• pnpm run i18n:audit
• pnpm run theme:color-audit:all
• git diff --check gcwing/main...HEAD