Playground: real markdown rendering, Safari tooltip fix, full screen, named files

[omdxp]

Summary

• fls hover/completion docs now render as real markdown (headings, lists, code blocks, clickable See-also links that jump in-editor) instead of raw text with literal syntax showing.
• Fixed a real, severe pre-existing bug: switching away from a file and back silently discarded all unsaved edits in that file (the cached CodeMirror state was never updated after a live edit).
• Fixed the hover/completion tooltip rendering clipped/hidden in Safari specifically - it uses absolute positioning there (unlike fixed everywhere else), and a redundant overflow:auto on the editor host was clipping it. Also raised the tooltip z-index well above the Playground's own overlays (full screen mode) so it can never render behind them in any browser.
• Fixed .pg-tree-btn (the entry/delete buttons in the file tree) not being theme-aware in light mode.
• Added a "Full screen" toggle for the editor pane (Esc to exit).
• "+ New" now asks for a filename (nested paths like lib/helpers.fn work) instead of auto-naming file1.fn/file2.fn.
• Added a small discoverability hint above the editor (goto-definition gesture, hover-for-docs, completions).

Test plan

• npx tsc --noEmit clean
• Production build (vite build) clean
• Verified directly via Playwright across Chromium and WebKit (Safari engine): markdown rendering, See-also link navigation + cursor focus restoration, file-switch content persistence, full screen toggle (including Escape to exit), named file creation (including nested paths and Escape-cancels-without-creating), light/dark theme parity, and the Safari-specific tooltip clipping fix (confirmed broken before the fix, confirmed fixed after, in the real WebKit engine)