fix(camera): fill the dead space below the 4:3 preview v1.105

On tall phones the 4:3 preview left a large black void between the
preview bottom and the shutter bar pinned to the screen bottom, which
looked broken.

- The bottom control bar (shutter + import) now fills the whole region
  below the preview: its top margin is set in code to topbar(56dp) +
  preview height, with match_parent height and center_vertical gravity,
  so the shutter sits balanced in that area instead of dropping to the
  very bottom. Background made solid black for a clean control panel.

Constraint: preview stays 4:3 (capture FOV unchanged); only the controls' placement is computed from the measured preview height.
Confidence: high
Scope-risk: local (camera layout/placement)
Reversibility: trivial
Directive: on device, confirm the shutter is comfortably centered below the preview across aspect ratios.
Tested: assembleDebug + testDebugUnitTest exit=0 (22 tests).
Not-tested: on-device across different screen ratios.
Related: user camera screenshot showing the black gap; v1.104 UI pass

🐙 Autopus <noreply@autopus.co>