fix(amu): enable gate controls only for Happy-Hare's selected gate

[gmmcosta15]

Description

• Feature
• Bug fix
• Code refactor
• Documentation

Last PR of the AMU stack (#345-#347 merged). Happy-Hare's bare MMU_LOAD, MMU_EJECT and MMU_CHECK_GATE act on the gate Happy-Hare has selected, so the AMU page now shows and enables controls only for that gate.

Changes

• The info panel follows Happy-Hare's selected gate (printer.mmu.gate).
  • With no gate selected (unknown or bypass) the panel clears, and only Unload can stay enabled.
  • Before, it kept showing a stale gate with live buttons.
• Button rules now match the AMUManager guards:
  • Load and Eject: MMU enabled, homed, filament unloaded, gate not EMPTY.
  • Check Gate: MMU enabled, homed, filament unloaded (allowed on an empty gate).
  • Unload: MMU enabled and filament not unloaded.
  • A disabled MMU greys out everything, including the material, temperature and colour fields.
• Load and Unload open the load popup only when the manager actually sent the command. Before, the popup always opened, and the slot raised when no popup was passed.
• Temperature and material edits no longer raise ValueError on the empty-field placeholder or on non-finite values; nothing is sent in those cases.
• The temperature numpad minimum is 1, because Happy-Hare treats TEMP=0 as "keep the current value".
• The carousel redraws only when the gates or the filament position change; the panel updates only when gate, enabled or homed change.
• SpoolCarousel.selectIndex(-1) now unchecks the highlighted button (an exclusive button group refuses to otherwise).
• Cleanup:
  • removed the unused request_gate_map signal;
  • removed the duplicate setObjectName calls; the page is now amu_page;
  • replaced the lambdas with named slots;
  • simplified the back-button rewiring (same behaviour).

Tests

• tests/widgets/test_amu_widgets_unit.py covers:
  • the button matrix for each gate and MMU state;
  • clearing the panel on bypass or when there is no state;
  • gate selection;
  • popup gating;
  • temperature parsing;
  • back-button rewiring;
  • redraw skipping.