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/shared/ipc.ts

@@ -33,7 +33,14 @@ export const IpcChannels = {
   errorLogList: 'errorlog:list',
   errorLogClear: 'errorlog:clear',
   backupExport: 'backup:export',
-  backupImport: 'backup:import'
+  backupImport: 'backup:import',
+  systemThemeGet: 'system-theme:get',
+  /** main → renderer push channel for OS theme/contrast changes (nativeTheme 'updated') */
+  systemThemeUpdate: 'system-theme:update',
+  openHighContrastSettings: 'shell:open-high-contrast-settings',
+  /** main → renderer push channel: a URL handed to AeroFetch from outside the app
+   *  (the aerofetch:// protocol, or a .url file via Explorer's "Send to" menu) */
+  externalUrl: 'external-url'
 } as const
 
 /** Sentinel format id meaning "let yt-dlp pick the best video+audio". */
@@ -69,6 +76,21 @@ export type CookieSource = 'none' | 'browser' | 'login'
 export const COOKIE_BROWSERS = ['chrome', 'edge', 'firefox', 'brave', 'opera', 'vivaldi'] as const
 export type CookieBrowser = (typeof COOKIE_BROWSERS)[number]
 
+/** Accent color presets for the brand ramp (see src/renderer/src/theme.ts). */
+export const ACCENT_COLORS = ['toffee', 'slate', 'evergreen', 'lavender'] as const
+export type AccentColor = (typeof ACCENT_COLORS)[number]
+
+/** UI color theme: an explicit mode, or 'system' to follow the OS preference. */
+export type ThemeMode = 'light' | 'dark' | 'system'
+
+/** OS-level theme signal, read from Electron's nativeTheme in the main process. */
+export interface SystemThemeInfo {
+  /** whether Windows is currently in dark mode */
+  shouldUseDarkColors: boolean
+  /** whether a Windows high-contrast theme is active */
+  shouldUseHighContrastColors: boolean
+}
+
 /** SponsorBlock segment categories (subset of yt-dlp's, the ones Seal exposes). */
 export const SPONSORBLOCK_CATEGORIES = [
   'sponsor',
@@ -270,8 +292,10 @@ export interface Settings {
   maxConcurrent: number
   /** yt-dlp output template, e.g. '%(title)s.%(ext)s' */
   filenameTemplate: string
-  /** UI color theme */
-  theme: 'light' | 'dark'
+  /** UI color theme: explicit light/dark, or 'system' to follow the OS */
+  theme: ThemeMode
+  /** brand accent preset (buttons, links, selected nav item) */
+  accentColor: AccentColor
   /** auto-detect video links from the clipboard on window focus */
   clipboardWatch: boolean
   /** default post-processing / format options for new downloads */
@@ -296,6 +320,8 @@ export interface Settings {
   defaultTemplateId: string | null
   /** show a native OS notification when a download finishes or fails */
   notifyOnComplete: boolean
+  /** false until the first-run welcome screen has been dismissed */
+  hasCompletedOnboarding: boolean
 }
 
 /**