Qt-removal R7.5a: 4 bundled mechanical parity fixes

[dovvnloading]

Summary

• Wires "Conversation Node" and "HTML Renderer" into execute_plugin — both node kinds have existed since R3.17/R3.25 with zero UI-reachable creation path; reuses the already-shipped add_conversation_node/add_html_node canvas methods with the same branch-point-child pattern as the other 6 real node-creation plugins.
• Adds "New Chat" and "Focus on Selection" to the command palette (both reuse existing capability — newChat's backend intent already existed, fitView/selection reads are already used elsewhere).
• Makes Export real on Chat and Code canvas nodes (previously a disabled "Export lands in R6" stub) — frontend-only download via a new shared downloadTextFile helper, matching ImageNodeView's existing Export pattern. Code node guesses a file extension from its language field, falling back to .txt.
• Corrects the Help panel's "Grid and Guide Controls" text, which falsely claimed smart guides, orthogonal routing, and faded connections are all available today — none of the three exist yet in this rewritten stack.

This is R7.5a of the Qt-removal plan's R7.5 phase (closing confirmed parity gaps ahead of the Qt cutover) — the smallest, most mechanical bundle of the 7 scoped sub-increments, chosen first since it reuses 100% existing, already-tested backend primitives.

Test plan

• backend/tests/test_plugins.py: new tests for both execute_plugin branches (no-parent, unknown-parent, real-creation), a monkeypatch-based test proving the generic fallback notice still works for a future unhandled plugin name, and an explicit check that every _PLUGINS entry now has a real branch
• commands.test.ts, ChatNodeView.test.tsx, CodeNodeView.test.tsx: new tests for the 2 commands and both Export actions
• Full verification: 864 backend / 1105 frontend tests passing, typecheck/lint/build clean, Qt-free burn-down gate unchanged
• One adversarial review pass (backend/frontend/integration) — zero confirmed defects, no fix pass needed
• Live-driven against a real create_app() instance over the real WS protocol (isolated scratch settings/chat-db files; real ~/.graphlink/session.dat SHA-256 confirmed unchanged before/after)