Complete Phase E theming, onboarding, and Windows share integration

Adds theme presets (Toffee/Slate/Evergreen/Lavender) with a "follow system"
mode and high-contrast awareness, a first-run welcome screen, and the
Windows analog of Android's share sheet for a Win32 app: an aerofetch://
protocol handler plus an Explorer "Send to AeroFetch" entry, both routed
through a single-instance lock so a second launch hands its link to the
already-running window instead of opening a duplicate.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/preload/index.ts

@@ -16,7 +16,8 @@ import {
   type CommandPreviewResult,
   type ErrorLogEntry,
   type BackupExportResult,
-  type BackupImportResult
+  type BackupImportResult,
+  type SystemThemeInfo
 } from '@shared/ipc'
 
 // The surface exposed to the renderer. Keep this thin: it only forwards to IPC.
@@ -99,6 +100,27 @@ const api = {
     const listener = (_e: IpcRendererEvent, ev: DownloadEvent): void => cb(ev)
     ipcRenderer.on(IpcChannels.downloadEvent, listener)
     return () => ipcRenderer.removeListener(IpcChannels.downloadEvent, listener)
+  },
+
+  getSystemTheme: (): Promise<SystemThemeInfo> => ipcRenderer.invoke(IpcChannels.systemThemeGet),
+
+  /** Subscribe to OS theme/contrast changes. Returns an unsubscribe function. */
+  onSystemThemeUpdate: (cb: (info: SystemThemeInfo) => void): (() => void) => {
+    const listener = (_e: IpcRendererEvent, info: SystemThemeInfo): void => cb(info)
+    ipcRenderer.on(IpcChannels.systemThemeUpdate, listener)
+    return () => ipcRenderer.removeListener(IpcChannels.systemThemeUpdate, listener)
+  },
+
+  /** Opens Windows' "Contrast themes" accessibility settings page. */
+  openHighContrastSettings: (): Promise<void> =>
+    ipcRenderer.invoke(IpcChannels.openHighContrastSettings),
+
+  /** Subscribe to links handed to AeroFetch from outside (aerofetch:// or a
+   *  "Send to" .url file). Returns an unsubscribe function. */
+  onExternalUrl: (cb: (url: string) => void): (() => void) => {
+    const listener = (_e: IpcRendererEvent, url: string): void => cb(url)
+    ipcRenderer.on(IpcChannels.externalUrl, listener)
+    return () => ipcRenderer.removeListener(IpcChannels.externalUrl, listener)
   }
 }