Reduce mobile toolbar wrapping and relocate fullscreen toggle

[vchelaru]

Problem

On mobile the top toolbar wraps to multiple rows (Run, Examples, Share, Files, Gist, "?", build timestamp, fullscreen). The exact wrap point varies by device, but we can reduce the likelihood by trimming the row.

Changes

Toolbar space savings (mobile-only, gated to max-width: 767px / xf-hide-mobile):

• Run/Restart/Stop → icon-only on mobile. The play/stop glyphs are universally recognizable; only the text labels are hidden (icons stay). Examples keeps its label — it's the primary first action for new users.
• Hide the "built …" version timestamp on mobile (dev/version info, low value on a phone).
• Fix the oversized About "?" control. It's an <a> (browser default box-sizing: content-box) while every other toolbar control is a <button> (border-box), so the mobile min-width/height: 44px touch-target rule added padding+border on top for the anchor — rendering it ~40% larger. Added box-sizing: border-box to the shared toolbar rule so it matches the buttons.

Fullscreen toggle relocation (all viewports):

• Moved the editor-collapse / game-fullscreen toggle out of the toolbar into a single canvas overlay button, pinned top-right (kept off the top-left, where game UI/HUD typically lives). One button covers both directions — expand game (screen-full) / restore editor (screen-normal) — faint at 0.5 opacity until hovered. Frees another toolbar slot and keeps the affordance on the window it acts on.

Notes

• dotnet build passes clean (0 warnings, 0 errors).
• Pure markup/CSS; no logic changes, no unit tests apply. Worth an e2e mobile-viewport check if desired.
• No e2e selectors referenced the removed toolbar fullscreen button.

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