docs(gui): add comprehensive Sphinx documentation for GUI

- Add getting-started guide with installation, launching, and features
- Add API backend developer docs (routes, services, database)
- Add frontend developer docs (components, pages, api-client)
- Add screenshots for all GUI pages
- Update homepage with GUI section and navigation
- Link existing GUI design docs to toctree
- Fix syntax highlighting warnings in design docs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ryanmccann1024

docs/_static/images/gui/README.txt

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+# GUI Screenshots
+
+This directory contains screenshots for the GUI documentation.
+
+## Required Screenshots
+
+Capture these screenshots from the running GUI at http://127.0.0.1:8765:
+
+| Filename | Page | What to Capture |
+|----------|------|-----------------|
+| `run-list-page.png` | RunListPage (`/`) | Multiple run cards with varied statuses (pending, running, completed, failed) |
+| `new-run-page.png` | NewRunPage (`/runs/new`) | Form with template dropdown expanded |
+| `run-detail-logs.png` | RunDetailPage | Logs tab showing log content |
+| `run-detail-artifacts.png` | RunDetailPage | Artifacts tab showing file browser |
+| `topology-page.png` | TopologyPage (`/topology`) | Network graph with a node selected |
+| `config-editor.png` | ConfigEditorPage (`/config`) | Editor showing INI content |
+| `codebase-architecture.png` | CodebaseExplorerPage (`/codebase`) | Architecture view with module cards |
+| `codebase-code.png` | CodebaseExplorerPage | Code view with file tree and code panel |
+| `settings-page.png` | SettingsPage (`/settings`) | Theme selection options |
+
+## Guidelines
+
+- Use a consistent browser window size (1280x800 recommended)
+- Capture only the content area, not browser chrome
+- Use light theme for consistent documentation appearance
+- Ensure no sensitive or personal data is visible
+- Save as PNG format
+
+## After Capturing
+
+Once screenshots are added, update the `[Screenshot: ...]` placeholders in
+`docs/getting-started/gui/features.rst` with actual image directives:
+
+```rst
+.. image:: /_static/images/gui/run-list-page.png
+   :alt: Run list page showing simulation runs
+   :width: 100%
+```