Add frontend-only Leaf GUI prototype

[WorkofAditya]

Motivation

• Provide a clean, professional, minimalist GUI prototype that surfaces all core Leaf workflows and sections so a backend can be connected later.
• Deliver a static, design-first frontend that documents expected UI elements (overview, staging, diffs, history, branches, recovery, remotes, integrity checks, and a command map) without changing backend logic.

Description

• Add a static frontend prototype with gui/index.html that implements a shell layout, sidebar navigation, hero, metrics, quick actions, changes/staging panels, diff preview, history timeline and graph, branch/merge cards, recovery controls, remotes layout, integrity panel, and a searchable command map.
• Add polished responsive styling in gui/styles.css including the Leaf color system, glass-like panels, responsive grids, mobile breakpoints, terminal preview, timeline/graph visuals, and toast interactions.
• Add lightweight frontend JavaScript in gui/script.js to copy CLI commands to clipboard, show toast feedback, filter the searchable command map, and keep nav links active via an IntersectionObserver.
• Document the frontend prototype in the README.md and keep the implementation intentionally unconnected to the Leaf CLI/backend for later integration.

Testing

• Ran python -m py_compile $(find Modules -name '*.py' -print) successfully to validate Python sources compiled without syntax errors.
• Parsed gui/index.html using Python's HTMLParser successfully to validate basic HTML structure.
• Ran node --check gui/script.js successfully to validate the script has no syntax errors.
• Ran the test suite with pytest -q which completed successfully (9 passed), and attempted npx --yes playwright --version for screenshot automation but it failed due to an npm registry 403 Forbidden in this environment.

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Codex Task