Add landing page

[milanofthe]

Adds a landing page at / in the style of the pathsim homepage; the editor moves to /editor.

• Hero with a live read-only mini editor tile showing the autosaved session (fallback: bundled example); clicking it opens the editor with that model, no restore prompt.
• Recent files section backed by the existing IndexedDB recents (File System Access API browsers).
• Guided tours and examples sections replace the welcome modal, which is removed; the editor home button links to the landing.
• Deep links keep working: /?model=, /?modelgh= and /?backend=flask redirect to /editor with the full query string.
• FlowCanvas gains a readonly/embedded preview mode with canvas-local fitView.
• Shared UI patterns (nav, panel header, section label, cards, action cards, separator, theme view transition) consolidated into the app.css component library and a shared utility.