Fullscreen button fix and docs update

[vk2amv]

The always-visible fullscreen control was obscuring part of the remote session in the top-right corner when not in fullscreen mode. This change relocates the fullscreen action to the Ctrl+Alt+Shift session menu, preserving functionality without overlaying the remote display.

Documentation has also been updated to describe the available in-session controls, including the clipboard panel, paste synchronization, fullscreen exit methods, and the new menu-based Fullscreen action.

[davekempe]

Thanks @vk2amv, the move into the Ctrl+Alt+Shift session menu makes sense. The floating corner button at 0.45 opacity is still 80px of permanent clutter for the majority of sessions where people don't manually toggle fullscreen, and the per-entry fullscreen_on_connect covers the "always fullscreen" case anyway. Happy with the code direction.

One pushback on the docs commit. The README and docs/web-sessions.md both reference a ? Shortcuts popover (**In-session shortcuts help** popover (\? Shortcuts`)/Click ? Shortcuts in the session view for an in-app shortcut reference popover`), but there's no such element in current main and this PR doesn't add one. The closest thing is the "Press Ctrl+Alt+Shift to toggle this panel" hint that already lives inside the clipboard panel itself.

Should the docs just drop the ? Shortcuts references and lean on the clipboard panel's existing in-panel hint?

Happy to merge once that bit is sorted. The rest of the docs (the Ctrl+Alt+Shift / Ctrl+V / Esc table, the disable_copy interaction note) all check out against the code.