fix(editor): match IDEA font weights in the light scheme (#35)

Root cause: the editor had no font weight settings, and the WebView rendering of JetBrains Mono looks lighter than IDEA (#32).
Fix (IDEA's Settings › Editor › Font › Typography Settings): configurable Main font weight and Bold font weight (defaults Regular 400 / Bold 700) via --fw-code / --fw-code-bold; colour-scheme bold/regular overrides still win; settings preview updates live. Replaces the Light-only 450/550 hard-coding.
Siblings covered: sticky lines, diff/merge, run console (ANSI bold), Find in Files, completion/parameter info/docs, database editor, debugger value popup, matched braces, terminal with "Use editor font".
Verified: scoped vitest, ide-core tests, in-app on macOS (settings UI, editor weight follows the setting end to end).

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/ui/SourcePreview.module.css

@@ -7,6 +7,7 @@
   overflow: auto;
   background: var(--bg-app);
   font-family: var(--font-mono);
+  font-weight: var(--fw-code);
   font-size: var(--fs-ui);
   line-height: 19px;
   color: var(--fg);