fix(ui): calculator polish — smart input steps, unified tech/MAT lock idiom, tech floor, speed-lock ceiling fix

- pkg/calc: DriveForSpeed treats restMass==0 as a valid ceiling-only
  case (every positive drive solves it), so locking the displayed
  speed of a D=1, W=A=S=C=0 ship is no longer a phantom "infeasible".
- ship-design-area: drive/weapons/shields/cargo inputs use a JS-driven
  smart step on ArrowUp/ArrowDown (0↔1 jump, otherwise ±0.1) and hide
  the native spinner so it cannot produce invalid (0, 1) values;
  armament keeps its native step 1.
- Tech and planet MAT cells follow the same lock idiom as goal-seek
  locks: open padlock (🔓) over the inherited value → click to open
  an input with a closed padlock (🔒). The padlock slot is always
  reserved, so the column width is stable.
- Tech overrides (design area and modernization target) are floored
  at the player's current tech on this turn — a lower value is
  flagged as invalid.

ui/frontend/tests/fake-core.ts

@@ -94,10 +94,13 @@ export function makeFakeCore(overrides: Partial<Core> = {}): Core {
 		weaponsForAttack: ({ targetAttack, weaponsTech }) =>
 			weaponsTech <= 0 || targetAttack < 0 ? null : targetAttack / weaponsTech,
 		driveForSpeed: ({ targetSpeed, driveTech, restMass }) => {
+			if (driveTech <= 0 || targetSpeed <= 0) return null;
 			const ceiling = 20 * driveTech;
-			if (driveTech <= 0 || targetSpeed <= 0 || targetSpeed >= ceiling) {
-				return null;
+			if (restMass <= 0) {
+				if (targetSpeed !== ceiling) return null;
+				return 1;
 			}
+			if (targetSpeed >= ceiling) return null;
 			return (targetSpeed * restMass) / (ceiling - targetSpeed);
 		},
 		shieldsForDefence: ({ targetDefence, shieldsTech, restMass }) => {