Scale stage 2: continuous uiScale setting + the (-) 100% (+) stepper

AppSettings.textSize (compact|default|large) becomes uiScale: number, 0.5-4
in 0.1 steps, default 1. One shared clampUiScale (next to the
effectiveConcurrency precedent) defines "valid" for the store, the service
and the stepper: clamp to range, snap to the step grid, strip float noise,
and map NaN/Infinity to 1.

- SettingsStore.load migrates legacy files (compact/default/large ->
  0.9/1/1.2, never over an explicit uiScale) and clamps ALWAYS, not just on
  migration — buildPreload writes this value into every window's root
  font-size, so a hand-edited settings.json (uiScale: 100) must not be able
  to render the UI unusable. Verified against 100, 0.001, -3, NaN, Infinity
  and float noise; migration verified against a real legacy file.
- SettingsService: applyUiScale sets the root font-size (16 * scale, removed
  entirely at 1 per the delete-when-default rule). Rides the existing
  settingsChanged broadcast, so open editor/manager windows follow live.
- buildPreload bakes the same font-size so the first paint is already scaled
  — without it every window opens at 100% and snaps.
- Both tab-bars: the 3-button Interface row becomes the stepper. Clicking the
  percentage resets to 100% (tooltip says so). i18n: compact/default/large
  dropped, smaller/larger/reset added, all 7 locales.

Verified live, win7 + modern: scale 1 is pixel-identical to the pre-work
baseline (opaque interior diff exactly 0); 0.5x is a real dense mode (~2x
rows); 2x scales title bar, tabs, rows, icons and controls together with text
contained in its boxes — which is what stage 1c's 7.css bridge bought; 4x is
enormous but coherent, and the win7 title-bar controls stay on-screen at the
default window size. The known caveat stands: no minimum window size exists,
so 4x in a small window is legal but cramped.

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/types.ts

@@ -100,8 +100,13 @@ export interface AppSettings {
 	/** Visible optional file-list columns (both panels; Name always shows). */
 	columns: ColumnKey[];
 	speedUnit: "MBps" | "Mbps";
-	/** Size of the panel interface: file list, path bar, header controls. */
-	textSize: "compact" | "default" | "large";
+	/**
+	 * Interface scale factor, 0.5–4 in 0.1 steps (shown as 50%–400%). Works by
+	 * setting the root font-size — every size in the UI is rem, so one lever
+	 * scales BOTH skins whole. Replaced the 3-tier `textSize`
+	 * (compact|default|large); SettingsStore.load migrates old files.
+	 */
+	uiScale: number;
 	/** Run transfer jobs one at a time, or several in parallel. */
 	transferMode: "single" | "multi";
 	/** How many jobs run at once when transferMode is "multi". */
@@ -127,7 +132,7 @@ export const DEFAULT_SETTINGS: AppSettings = {
 	iconColors: "multi",
 	columns: ["size", "mtime"],
 	speedUnit: "MBps",
-	textSize: "default",
+	uiScale: 1,
 	transferMode: "single",
 	transferConcurrency: 3,
 	autoClearCompleted: true,
@@ -136,6 +141,23 @@ export const DEFAULT_SETTINGS: AppSettings = {
 	// autoRefresh: true,
 };
 
+export const UI_SCALE_MIN = 0.5;
+export const UI_SCALE_MAX = 4;
+export const UI_SCALE_STEP = 0.1;
+
+/**
+ * Clamp + snap a scale factor to the legal range and step grid. Shared by the
+ * store (a hand-edited settings.json must not be able to render the UI
+ * unusable), the settings service and the stepper — one definition of "valid",
+ * same as effectiveConcurrency below.
+ */
+export function clampUiScale(value: number): number {
+	if (!Number.isFinite(value)) return 1;
+	const clamped = Math.min(UI_SCALE_MAX, Math.max(UI_SCALE_MIN, value));
+	// snap to the step grid; toFixed strips float noise (0.7000000000000001 -> 0.7)
+	return Number((Math.round(clamped / UI_SCALE_STEP) * UI_SCALE_STEP).toFixed(2));
+}
+
 export function effectiveConcurrency(settings: AppSettings): number {
 	if (settings.transferMode === "single") return 1;
 	return Math.max(1, Math.min(8, Math.round(settings.transferConcurrency) || 1));