ifcviewer: first-person / fly view — share the desktop fly camera with web

DRY the fly camera into ViewportCore so desktop and web share identical math:
- flyMove(fwd,back,right,left,up,down,boost,dt): WASD in the view plane, QE
  along world +Z, forward = eye→target (no snap after orbiting), Shift = 5x,
  dt clamped to 0.1s. flyLook(dx,dy): turn in place (yaw/pitch, eye pinned,
  0.2 deg/px, pitch ±89.9). flyAdjustSpeed(notches): wheel scales speed x1.25.
  fly_move_speed_ + orbitEye now live in the core (orbitEye's duplicate removed
  from ViewportWindow).
- Desktop ViewportWindow: fpsIntegrate / mouse-look / wheel-speed call the core
  methods; behaviour unchanged, BonsaiViewer builds clean.
- Web main_web: Shift+F (or the Fly toolbar button) enters and pointer-locks the
  canvas; W/A/S/D/Q/E + Shift are held-tracked (keydown/keyup) and integrated
  each RAF frame with wall-clock dt; pointer-lock mouse deltas drive flyLook;
  wheel tunes speed. Exit on Esc OR a canvas click (matches desktop) — a
  pointerlockchange handler catches the browser eating the first Esc to release
  the lock (so a single Esc exits), guarded by fly_locked so a denied lock on
  entry doesn't insta-exit. Fly button reflects/ syncs active state.

Verified: web enter→WASD moves→click/Esc exits, 0 GPU errors; 113/113 desktop
core + 6/6 web smoke; desktop app builds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -779,12 +779,8 @@ private:
     Stopwatch    fps_last_tick_;
     Eigen::Vector2i       fps_press_center_;
     bool         fps_ignore_next_mouse_move_  = false;
-    // Fly base speed in m/s at no-modifier (Shift gives a 5× boost). Default
-    // 5.0 matches GL fps_move_speed_. Scrollwheel in fly mode adjusts this
-    // by ×1.25 / ×0.8 per notch, Blender-style — wheel does NOT zoom while
-    // in fly mode (which would change camera_distance_ underneath us and
-    // make speed jitter if speed were distance-scaled).
-    float        fps_move_speed_              = 5.0f;
+    // Fly base speed (m/s) + wheel adjustment now live in ViewportCore
+    // (fly_move_speed_ / flyAdjustSpeed), shared with the web fly path.
     // Per-frame [fly] dt log when WGPU_FLY_DEBUG=1. Diagnoses stutter:
     // print dt of each fpsIntegrate call and the prior render's elapsed
     // ms. Off by default (env-gated) so the normal log stays clean.