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/src/lib/sidebar/calculator-tab.svelte

@@ -14,7 +14,7 @@ switch (the inspector auto-opens on a planet click) — the calculator is a
 long-lived planning tool. `ensureGame` resets it when the game changes.
 -->
 <script lang="ts">
-	import { getContext } from "svelte";
+	import { getContext, tick } from "svelte";
 	import { appScreen } from "$lib/app-nav.svelte";
 
 	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
@@ -273,6 +273,18 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		cs.matValue < 0 ? i18n.t("game.calculator.invalid.negative") : "",
 	);
 
+	// Modernization target tech mirrors the design-area floor: a target
+	// below the player's current tech on this turn is meaningless (no
+	// upgrade), so flag it the same way.
+	function targetTechError(key: TechKey): string {
+		const value = cs.targetTech[key];
+		if (value < 0) return i18n.t("game.calculator.invalid.negative");
+		if (value < playerTech[key]) {
+			return i18n.t("game.calculator.invalid.tech_below_current");
+		}
+		return "";
+	}
+
 	// Locking a speed back-solves the drive block; with a zero drive the
 	// ship is deliberately immobile, so disallow it.
 	function lockDisabledReason(output: LockableOutputId): string {
@@ -291,8 +303,12 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	function onResetTech(key: TechKey): void {
 		cs.techOverridden[key] = false;
 	}
-	function onMatInput(): void {
+	const matInputRef: { el?: HTMLInputElement } = {};
+	async function activateMatOverride(): Promise<void> {
 		cs.matOverridden = true;
+		await tick();
+		matInputRef.el?.focus();
+		matInputRef.el?.select();
 	}
 	function resetMat(): void {
 		cs.matOverridden = false;
@@ -485,6 +501,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		resolved={resolvedCeil}
 		bind:techs={cs.techValues}
 		techOverridden={cs.techOverridden}
+		techFloor={playerTech}
 		computedInput={result.computedInput}
 		{onTechInput}
 		{onResetTech}
@@ -589,17 +606,17 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 				<div class="rrow">
 					<span class="label">{i18n.t("game.calculator.planet.mat")}</span>
 					<span class="cell">
-						<input
-							type="number"
-							step="0.01"
-							min="0"
-							bind:value={cs.matValue}
-							oninput={onMatInput}
-							aria-invalid={matError !== "" ? "true" : "false"}
-							title={matError}
-							data-testid="calculator-planet-mat"
-						/>
 						{#if cs.matOverridden}
+							<input
+								bind:this={matInputRef.el}
+								type="number"
+								step="0.01"
+								min="0"
+								bind:value={cs.matValue}
+								aria-invalid={matError !== "" ? "true" : "false"}
+								title={matError}
+								data-testid="calculator-planet-mat"
+							/>
 							<button
 								type="button"
 								class="lock active"
@@ -610,6 +627,23 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 							>
 								🔒
 							</button>
+						{:else}
+							<span
+								class="mat-val"
+								data-testid="calculator-planet-mat-value"
+							>
+								{cs.matValue}
+							</span>
+							<button
+								type="button"
+								class="lock"
+								title={i18n.t("game.calculator.mat.override")}
+								aria-label={i18n.t("game.calculator.mat.override")}
+								data-testid="calculator-mat-override"
+								onclick={() => void activateMatOverride()}
+							>
+								🔓
+							</button>
 						{/if}
 					</span>
 					<span></span>
@@ -638,18 +672,17 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 				<span class="col-head">{i18n.t("game.calculator.modern.cost")}</span>
 			</div>
 			{#each modernCosts?.perBlock ?? [] as row (row.key)}
+				{@const targetError = targetTechError(row.key)}
 				<div class="rrow">
 					<span class="label">{i18n.t(`game.calculator.field.${row.key}` as TranslationKey)}</span>
 					<span class="cell">
 						<input
 							type="number"
 							step="0.001"
-							min="0"
+							min={playerTech[row.key]}
 							bind:value={cs.targetTech[row.key]}
-							aria-invalid={cs.targetTech[row.key] < 0 ? "true" : "false"}
-							title={cs.targetTech[row.key] < 0
-								? i18n.t("game.calculator.invalid.negative")
-								: ""}
+							aria-invalid={targetError !== "" ? "true" : "false"}
+							title={targetError}
 							data-testid={`calculator-target-${row.key}`}
 						/>
 					</span>
@@ -899,4 +932,15 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		font-size: 0.8rem;
 		color: var(--color-accent);
 	}
+	/* Plain-text view of the planet MAT (mirrors `.tech-val` in the
+	   design area) so the cell width stays the same whether the value is
+	   the inherited planet number or the player's override. */
+	.mat-val {
+		flex: 1;
+		min-width: 0;
+		font-size: 0.85rem;
+		font-variant-numeric: tabular-nums;
+		text-align: right;
+		padding: 0.15rem 0.3rem;
+	}
 </style>