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/i18n/en.ts

@@ -57,7 +57,7 @@ export const en = {
 			teal: "Teal",
 		},
 		fileIcons: { label: "File icons", plain: "Plain", multi: "Multi" },
-		interface: { label: "Interface", compact: "Compact", default: "Default", large: "Large" },
+		interface: { label: "Interface", smaller: "Smaller", larger: "Larger", reset: "Reset to 100%" },
 		speedUnit: { label: "Speed unit" },
 		transfers: { label: "Transfers", single: "Single", multi: "Multi" },
 		atOnce: "At once",