docs: clarify Theme Switcher SSH setup

[jdtimothy]

Summary

• clarify that the Theme Switcher page targets the native Hermes Desktop app, not the separate hermes dashboard plugin SDK
• split installation instructions between the remote backend host and local Desktop computer, including the Windows %LOCALAPPDATA%\hermes location
• document that plugin API routes mount when the backend starts and require a reconnect/restart after enablement
• document the verified nonce-aware recovery boundary for stale Desktop-over-SSH backends and warn against killing the backend PID without reconciling its lock/token ownership state
• add regression coverage for the support and installation guidance

Verified reproduction and resolution

In the reported Desktop-over-SSH setup, an old Desktop-owned backend did not contain the Theme Switcher routes. After nonce-aware stale-backend recovery, Desktop created a fresh backend. Its OpenAPI document exposed all five /api/plugins/theme-switcher/* routes, and the Themes tab loaded and switched themes successfully.

This PR is documentation and regression coverage only; it does not change plugin runtime behavior or add a web-dashboard frontend.

Addresses #2.

Test plan

• uv run pytest tests/ -q — 9 passed
• uv run python scripts/generate_themes.py with no generated-file drift
• git diff --check

[CliffWade]

Merged, thank you! This genuinely clarified things. The old README said 'desktop browser' and assumed one machine, which was exactly how it looked before your report. I validated the Windows path against the app source (%LOCALAPPDATA%\hermes is the real default), resolved the one test-placement conflict with the cards PR that landed in between, and the full suite is green at 11 tests. Appreciate the detailed writeup and the regression test.