control.html: inject combo viewer panels; extract camera_panel to shared macro

Move camera_panel macro to macros/viewer_panel.html and import it in
both viewer.html and control_panel.html. Replace the old <template>
clone approach in control.html with three pre-rendered combo panels
(combo1/2/3) toggled via d-none on mode switch. initViewer() wires up
each panel's FitsViewer on page load. control.js loses the FitsViewer
import; switchViewer() becomes a simple show/hide by data-combo-view.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>