fix(ui): F8-06 calculator polish — always 3-decimal display, mono font, input cap

Owner feedback round 2 on PR #61:

- Pad every read-only calculator value to three decimals: tech labels,
  derived results (mass, speed, attack, defence, bombing, cargo
  capacity), planet MAT, planet build-rate, modernization cost, and
  the full-cargo capacity label all read as "1.000" instead of "1",
  matching the goal-seek back-solved input and the report. Drops
  thousands grouping so the same `fmt()` string also embeds cleanly
  in the read-only `<input type="number">` cell.
- Switch label and input styling onto the existing `--font-mono`
  token (right-aligned, tabular-nums) so columns line up vertically
  across rows like a financial table.
- Refuse a fourth decimal as the user types in every calculator
  number input (DWSC blocks, tech, MAT, custom load, lock value,
  modernization target tech): the `oninput` truncates the input text
  past three decimal digits and explicitly writes the truncated
  value back through `bind:value`, so Svelte's later reactive flush
  cannot undo the cap.
- Doc + tests follow the rule (five new vitest cases covering the
  3-decimal label format, the input cap on each input class, and
  the integer-padding rule for derived results).

ui/frontend/src/lib/calculator/ship-design-area.svelte

@@ -139,6 +139,20 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		const floor = techFloor[key];
 		techs[key] = next < floor ? floor : next;
 	}
+	// Refuse a fourth decimal as typing happens: keeps the calculator
+	// from ever displaying a >3-decimal fraction the user could not
+	// have intended (the calculator math is `Ceil3`-rounded for display
+	// anyway). Pairs with `bind:value` — `apply` overwrites the bound
+	// state when Svelte's own bind handler has already read the
+	// over-precise number.
+	function capDecimals(event: Event, apply: (next: number) => void): void {
+		const el = event.currentTarget as HTMLInputElement;
+		const txt = el.value;
+		const dot = txt.indexOf(".");
+		if (dot < 0 || txt.length - dot - 1 <= 3) return;
+		el.value = txt.slice(0, dot + 4);
+		apply(el.valueAsNumber);
+	}
 
 	const BLOCK_ROWS: {
 		key: keyof DesignBlocksState;
@@ -196,6 +210,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 					title={blockError(row.key)}
 					data-testid={`calculator-block-${row.key}`}
 					onkeydown={(e) => onBlockKey(e, row.key, row.smartStep)}
+					oninput={(e) => capDecimals(e, (v) => (blocks[row.key] = v))}
 				/>
 			{/if}
 			{#if row.tech !== null}
@@ -213,6 +228,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 							title={techError(techKey)}
 							data-testid={`calculator-tech-${techKey}`}
 							onkeydown={(e) => bumpTech(e, techKey)}
+							oninput={(e) => capDecimals(e, (v) => (techs[techKey] = v))}
 						/>
 						<button
 							type="button"
@@ -274,7 +290,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		font-size: 0.8rem;
 	}
 	input {
-		font: inherit;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
 		width: 100%;
 		min-width: 0;
@@ -284,6 +300,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		border: 1px solid var(--color-border);
 		border-radius: 3px;
 		font-variant-numeric: tabular-nums;
+		text-align: right;
 	}
 	/* Hide native spinners across the design area — the row drives
 	   every numeric edit through ArrowUp/ArrowDown so the column
@@ -313,6 +330,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 	.tech-val {
 		flex: 1;
 		min-width: 0;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
 		font-variant-numeric: tabular-nums;
 		text-align: right;