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>

ui/src/app/core/settings.service.ts

@@ -1,6 +1,6 @@
 import { Injectable, computed, inject, signal } from '@angular/core';
 import { resolveLocale } from '@shared/i18n';
-import { DEFAULT_SETTINGS, type AppSettings, type ColumnKey } from '@shared/types';
+import { clampUiScale, DEFAULT_SETTINGS, type AppSettings, type ColumnKey } from '@shared/types';
 import { I18nService } from './i18n.service';
 import { RpcService } from './rpc.service';
 
@@ -14,7 +14,7 @@ export class SettingsService {
   readonly iconColors = signal<AppSettings['iconColors']>(DEFAULT_SETTINGS.iconColors);
   readonly columns = signal<ColumnKey[]>(DEFAULT_SETTINGS.columns);
   readonly speedUnit = signal<AppSettings['speedUnit']>(DEFAULT_SETTINGS.speedUnit);
-  readonly textSize = signal<AppSettings['textSize']>(DEFAULT_SETTINGS.textSize);
+  readonly uiScale = signal<number>(DEFAULT_SETTINGS.uiScale);
   readonly transferMode = signal<AppSettings['transferMode']>(DEFAULT_SETTINGS.transferMode);
   readonly transferConcurrency = signal(DEFAULT_SETTINGS.transferConcurrency);
   readonly autoClearCompleted = signal(DEFAULT_SETTINGS.autoClearCompleted);
@@ -58,7 +58,7 @@ export class SettingsService {
     this.iconColors.set(settings.iconColors);
     this.columns.set(settings.columns);
     this.speedUnit.set(settings.speedUnit);
-    this.textSize.set(settings.textSize);
+    this.uiScale.set(settings.uiScale);
     this.transferMode.set(settings.transferMode);
     this.transferConcurrency.set(settings.transferConcurrency);
     this.autoClearCompleted.set(settings.autoClearCompleted);
@@ -70,7 +70,7 @@ export class SettingsService {
     this.applyTheme();
     this.applyDarkTone();
     this.applyAccent();
-    this.applyTextSize();
+    this.applyUiScale();
     this.applyLocale();
   }
 
@@ -140,9 +140,9 @@ export class SettingsService {
     void this.persist();
   }
 
-  setTextSize(size: AppSettings['textSize']): void {
-    this.textSize.set(size);
-    this.applyTextSize();
+  setUiScale(scale: number): void {
+    this.uiScale.set(clampUiScale(scale));
+    this.applyUiScale();
     void this.persist();
   }
 
@@ -204,12 +204,17 @@ export class SettingsService {
     }
   }
 
-  private applyTextSize(): void {
-    const size = this.textSize();
-    if (size === 'default') {
-      delete document.documentElement.dataset['text'];
+  /** The whole interface scale is ONE declaration: every size in the app is
+   *  rem, so moving the root font-size moves both skins in lockstep. 1 = the
+   *  browser-default 16px root; delete-when-default keeps the pristine state
+   *  attribute-free like the other axes. buildPreload mirrors this for a
+   *  flash-free first paint. */
+  private applyUiScale(): void {
+    const scale = this.uiScale();
+    if (scale === 1) {
+      document.documentElement.style.removeProperty('font-size');
     } else {
-      document.documentElement.dataset['text'] = size;
+      document.documentElement.style.fontSize = `${16 * scale}px`;
     }
   }
 
@@ -227,7 +232,7 @@ export class SettingsService {
       iconColors: this.iconColors(),
       columns: this.columns(),
       speedUnit: this.speedUnit(),
-      textSize: this.textSize(),
+      uiScale: this.uiScale(),
       transferMode: this.transferMode(),
       transferConcurrency: this.transferConcurrency(),
       autoClearCompleted: this.autoClearCompleted(),