Adds a 'fit' toolbar toggle on the /screen VNC viewer. When on (the
default), the canvas is CSS-scaled down to fit the browser viewport
preserving aspect ratio — no more scrolling a desktop larger than the
window. When off, the canvas renders at native resolution with scroll.
The choice persists in localStorage.
The canvas's intrinsic resolution is never touched — only its display
size. sendPointer now rescales client coordinates by the canvas
display-vs-intrinsic ratio so clicks stay accurate in fit mode (this
also fixes a latent off-by-scale bug). Toolbar buttons share a .tbtn
class for consistent styling.
Issue #133 (fit toggle now; dynamic desktop resize tracked separately).
305a32220b