Add Missile Silo UX widget

A movable, factory-queue-styled strip for missile silos that aggregates
all owned silos into one panel, with a button per missile type showing
ready / queued / build progress and a shared slots meter.

- Left-click a type: arm fire-mode (a real hidden engine command, so the
  guihandler owns mutual-exclusion and map/UI hit-testing); click again,
  or when none are ready, to queue one instead.
- Middle-click: queue one at the best-placed silo. Right-click: cancel one
  (never destroys a finished missile).
- Hover or arm a type to preview each silo's range ring (solid/bright when
  that silo can fire it, dim/dashed otherwise) plus the AoE falloff at the
  cursor, mirroring the stock Attack-AoE indicator.

The panel auto-hides when you own no silos. Fire dispatches CMD.ATTACK to
the specific ready missile of the chosen type, matching cmd_missile_silo's
force-fire semantics (ground point, or unit-tracking for the homing Zeno).

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

Author: danfireman