Support custom editor font weights in steps of 50 (#74)

JetBrains Mono at 500 looks heavier than PyCharm's rendering, and 450 was rounded up to 500. Adds a Custom option with an input to the Main and Bold font weight rows; weights are clamped to 100–900 and rounded to the nearest 50 (the terminal, which follows the editor weights, gets the same steps).

Verified in the app: Custom → 450 shows the input and the preview renders at 450. Maintainer follow-up: formatted the dialog test.

Related: #32

https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/platform/colorScheme.test.ts

@@ -60,9 +60,10 @@ describe("color schemes", () => {
     const d = validateEditorExtras({});
     expect([d.fontWeight, d.boldFontWeight]).toEqual([400, 700]);
     const v = validateEditorExtras({ fontWeight: 349, boldFontWeight: 1200 });
-    expect([v.fontWeight, v.boldFontWeight]).toEqual([300, 900]);
+    expect([v.fontWeight, v.boldFontWeight]).toEqual([350, 900]);
     expect(validateEditorExtras({ fontWeight: "bold" }).fontWeight).toBe(400);
     expect(fontWeightVars(500, undefined)).toEqual({ "--fw-code": "500", "--fw-code-bold": "700" });
+    expect(fontWeightVars(450, 650)).toEqual({ "--fw-code": "450", "--fw-code-bold": "650" });
   });
 
   it("coerces editor settings to their defaults' shape", () => {

src/platform/editorSettings.ts

@@ -205,10 +205,10 @@ export const FONT_WEIGHTS: ReadonlyArray<readonly [number, string]> = [
   [900, "Black"],
 ];
 
-/** Nearest CSS weight step (100–900); `def` for anything that is not a number. */
+/** CSS weight from 100 to 900, rounded to the nearest 50. */
 export function fontWeightOf(raw: unknown, def: number): number {
   if (typeof raw !== "number" || !Number.isFinite(raw)) return def;
-  return Math.min(900, Math.max(100, Math.round(raw / 100) * 100));
+  return Math.min(900, Math.max(100, Math.round(raw / 50) * 50));
 }
 
 /** `--fw-code` / `--fw-code-bold`: what editor-font text and its bold runs render with. */