Fix: AMS drying popover no longer renders off the bottom of the viewport (#1447 part 1)

The flame-icon onClick on PrintersPage computed popover position as a
  fixed { top: rect.bottom + 4, left: Math.max(8, rect.right - 240) }
  with no viewport-overflow check. The flame icon sits at the bottom of
  the AMS info section on the printer card, so on most realistic viewports
  rect.bottom + 4 + popover_height (~320px) overruns viewport.height and
  the popover renders partially or entirely off-screen with the Start
  button unreachable. Reporter (kleinweby, P1S + AMS-HT) worked around it
  via DevTools to confirm the popover was actually there, just clipped.

  Extract a computePopoverPosition() helper in utils/popoverPosition.ts:
  - defaults to below + right-aligned to the trigger (preserves the
    original visual layout when there's room),
  - flips ABOVE the trigger when below would overflow AND above fits,
  - stays below in the degraded case (popover taller than the viewport) —
    at least the top is visible and the user can scroll inside the
    popover; flipping to a top-clipped position would lose the action
    buttons too,
  - clamps the left coordinate so a trigger near either viewport edge
    can't push the popover off-screen horizontally either.

  Both PrintersPage callsites (the compact AMS row at :3498 and the
  dual-nozzle layout at :4011) route through the helper.

  This is part 1 of #1447. The functional drying bug — printer receives
  the ams_filament_drying MQTT command, ACKs it, but never starts/stops
  drying on Bambuddy's request while the printer's own touchscreen works
  — is NOT addressed here. Diagnosing it needs the printer's actual
  response payload (whether result: "fail" and the specific reason code),
  which bambu_mqtt.py:918 currently doesn't log for ams_filament_drying.
  Punted to a follow-up where I'll extend the existing extrusion_cali_* /
  ams_filament_setting payload-logging path at :919-920 to cover
  ams_filament_drying too, ask the reporter to retry, and fix the
  command side based on what the printer actually returns.

frontend/src/utils/popoverPosition.ts

@@ -0,0 +1,82 @@
+export interface PopoverPosition {
+  top: number;
+  left: number;
+}
+
+interface RectLike {
+  top: number;
+  bottom: number;
+  left: number;
+  right: number;
+}
+
+export interface ComputePopoverPositionOpts {
+  /** Trigger element's bounding rect (viewport coordinates). */
+  triggerRect: RectLike;
+  /** Popover width in CSS pixels. */
+  popoverWidth: number;
+  /**
+   * Estimated popover height in CSS pixels. Used to detect bottom-edge
+   * overflow so we can flip above the trigger. A conservative over-estimate
+   * is preferable to an under-estimate — over-estimating just flips slightly
+   * sooner, under-estimating leaves the popover clipped off the viewport.
+   */
+  estimatedHeight: number;
+  /** Viewport height. Defaults to window.innerHeight. Injectable for tests. */
+  viewportHeight?: number;
+  /** Viewport width. Defaults to window.innerWidth. Injectable for tests. */
+  viewportWidth?: number;
+  /** Margin to keep between the popover and the viewport edges. */
+  margin?: number;
+  /** Gap between the trigger and the popover. */
+  gap?: number;
+}
+
+/**
+ * Compute fixed-positioning coordinates for a popover anchored to a trigger.
+ *
+ * Default placement is BELOW the trigger, right-aligned to the trigger. Flips
+ * to ABOVE the trigger when below would overflow the viewport (#1447 — the
+ * AMS drying popover on the printer card sits at the bottom of the AMS row
+ * and was rendering off the bottom of the viewport with the Start button
+ * unreachable on smaller screens).
+ *
+ * Horizontal axis right-aligns to triggerRect.right and clamps to the
+ * viewport with the configured margin so a trigger near the right edge
+ * doesn't push the popover off-screen.
+ */
+export function computePopoverPosition(opts: ComputePopoverPositionOpts): PopoverPosition {
+  const {
+    triggerRect,
+    popoverWidth,
+    estimatedHeight,
+    viewportHeight = window.innerHeight,
+    viewportWidth = window.innerWidth,
+    margin = 8,
+    gap = 4,
+  } = opts;
+
+  // Vertical: prefer below, flip to above only when below overflows AND
+  // above would actually fit. If neither fits (a popover taller than the
+  // viewport), stay below — at least the top of the popover is visible
+  // and the user can scroll inside it, which is better than flipping to a
+  // top-clipped position where the action buttons might also be unreachable.
+  let top = triggerRect.bottom + gap;
+  const wouldOverflowBottom = top + estimatedHeight > viewportHeight - margin;
+  if (wouldOverflowBottom) {
+    const aboveTop = triggerRect.top - gap - estimatedHeight;
+    if (aboveTop >= margin) {
+      top = aboveTop;
+    }
+  }
+
+  // Horizontal: right-align to trigger; clamp to viewport bounds.
+  let left = triggerRect.right - popoverWidth;
+  if (left < margin) {
+    left = margin;
+  } else if (left + popoverWidth > viewportWidth - margin) {
+    left = Math.max(margin, viewportWidth - popoverWidth - margin);
+  }
+
+  return { top, left };
+}