Address Gadfly review on the units fix

Five findings, all fair:

The soft-minimum hint contradicted itself. "did you mean ft?" made sense when
the grid field was labelled inches sitting under a feet field — which is exactly
what this PR removed. The remaining failure mode is different: someone reaching
for plant spacing, which belongs to the bed, not the garden. The hint now says
that, and points at where the bed grid actually lives. The stale "1 typed
meaning one foot" comments went with it.

gridNote was a three-level nested ternary; it is now gridNoteFor, with the same
logic in explicit branches and the reasoning in one doc comment rather than
threaded between the arms.

cmFromDisplay(parseFloat(gridSize), unit) was computed once for the hint and
again in the submit handler. Hoisted to a single per-render value so the two
can't disagree about what was entered.

roundCm's comment called the 1e-6 cm grain "1 nanometer"; it is 10 nm. Corrected
— the constant was always fine, only the comment was wrong.

Extracted the duplicated overlay-badge class string.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/lib/units.ts

@@ -12,9 +12,9 @@ const INCHES_PER_FOOT = 12
 export const MIN_DIMENSION_CM = 1
 export const MAX_DIMENSION_CM = 10_000
 
-// A garden grid finer than this at *dimension* scale is a mis-entry, not a
-// preference (typing "1" in a feet field meaning one foot, and landing on a 1 cm
-// grid). Soft: the form warns, it doesn't refuse.
+// Below this, a garden-scale grid is fine enough to be worth questioning — it
+// usually means someone reached for plant spacing, which belongs to the bed, not
+// the garden. Soft: the form hints, it doesn't refuse.
 export const MIN_GARDEN_GRID_CM = 10
 
 /** Whether a centimeter dimension is within the server's accepted range. */
@@ -23,9 +23,9 @@ export function isValidDimensionCm(cm: number): boolean {
 }
 
 /** Round away IEEE-754 dust without rounding away precision: 12 × 2.54 is
- *  30.479999999999997 in binary floating point, and 1 nanometer is far below
- *  anything a garden cares about. Every cm column is REAL, so entry stays exact
- *  — this only stops 30.48 from being stored as 30.479999999999997. */
+ *  30.479999999999997 in binary floating point, and the 1e-6 cm (10 nm) grain
+ *  here is far below anything a garden cares about. Every cm column is REAL, so
+ *  entry stays exact — this only stops 30.48 being stored as 30.479999999999997. */
 function roundCm(cm: number): number {
   return Math.round(cm * 1e6) / 1e6
 }