Split 'r' hotkey: fold-only vs full view reset

[Copilot]

The r hotkey conflated two distinct operations: folding the brain and resetting camera position — while not even resetting zoom, making it neither a clean fold nor a true reset.

Changes

• r key — now only folds the brain (mix → 0) without touching camera target, azimuth, altitude, or radius
• t key (new) — full view reset: folds brain + resets camera to defaults (target=[0,0,0], azimuth=45°, altitude=75°, radius=400)
• Docs — updated keyboard shortcut table and Camera Controls section in docs/userguide/webgl.rst

Relevant diff (mriview.js)

// r: fold only (no camera movement)
var fold_brain = function() {
    this.animate([{state:'mix', idx:parseFloat(viewopts.anim_speed), value:0}]);
}.bind(this);

// t: full reset
this.reset_view = function() {
    this.animate([
        {state:'camera.target',   idx:parseFloat(viewopts.anim_speed), value:[0,0,0]},
        {state:'camera.azimuth',  idx:parseFloat(viewopts.anim_speed), value:45},
        {state:'camera.altitude', idx:parseFloat(viewopts.anim_speed), value:75},
        {state:'camera.radius',   idx:parseFloat(viewopts.anim_speed), value:400},
        {state:'mix',             idx:parseFloat(viewopts.anim_speed), value:0},
    ]);
}.bind(this);

[mvdoc]

@copilot update the help window too if necessary

[Copilot]

@copilot update the help window too if necessary

The help window is auto-generated by iterating over the UI descriptor objects (including this.ui._desc.camera._desc). Since both new entries already have key and help properties — fold: {action:fold_brain, key:'r', help:'Fold brain'} and reset: {action:this.reset_view, key:'t', help:'Reset view'} — they will appear correctly in the help overlay without any additional changes needed.