fix: luminance-aware chrome theming for light themes; serialized settings writes; buffered log flush

- Derive tab-bar/chrome palette by background luminance: light themes keep
  a near-background bar with black-tinted tab overlays instead of a muddy
  gray strip; dark themes unchanged
- Theme dockview tabs via the group-scoped --dv-*-tab-* vars its own rules
  consume (they outspecify our .dv-tab rules and leaked abyss navy onto
  light tabs); bump inactive-tab hover specificity to match
- Convert settings dialog + highlight editor hardcoded white text/border
  tints to color-mix over --chrome-fg so panes stay readable on light themes
- commands.ts: per-file log write buffer with a single drain loop per file
  (burst output coalesces into one appendFile per IO tick, chain no longer
  grows per logWrite); stop-tail rides the same buffer
- settingsStore: serialize all writers through mutateSettings() queue that
  re-reads latest state per mutation (tray close-action vs settings UI full
  saves no longer clobber each other)
- tests: burst ordering + stop-tail case for the log buffer

src/main/tray.ts

@@ -1,6 +1,6 @@
 import { app, BrowserWindow, dialog, Menu, nativeImage, Tray } from 'electron'
 import type { SystemSettings } from '@shared/settings'
-import { loadSettings, saveSettings } from './settingsStore'
+import { loadSettings, mutateSettings } from './settingsStore'
 import trayIconPath from './assets/tray.png?asset'
 
 export type CloseAction = NonNullable<SystemSettings['closeAction']>
@@ -15,9 +15,12 @@ export function markQuitting(): void {
 }
 
 function persistCloseAction(action: CloseAction): void {
-  const next = loadSettings()
-  next.system.closeAction = action
-  saveSettings(next)
+  // Serialized read-modify-write on the latest state: must not stomp (or be
+  // stomped by) a concurrent full-snapshot save from the settings UI.
+  void mutateSettings((current) => {
+    current.system.closeAction = action
+    return current
+  })
 }
 
 function showMainWindow(showOrCreate: () => void): void {