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/main/settings.ts

@@ -7,6 +7,7 @@ import {
   VIDEO_CODECS,
   SPONSORBLOCK_CATEGORIES,
   COOKIE_BROWSERS,
+  ACCENT_COLORS,
   DEFAULT_DOWNLOAD_OPTIONS,
   type Settings,
   type DownloadOptions,
@@ -21,6 +22,7 @@ const DEFAULTS: Settings = {
   maxConcurrent: 2,
   filenameTemplate: '%(title)s.%(ext)s',
   theme: 'light',
+  accentColor: 'toffee',
   clipboardWatch: true,
   downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
   proxy: '',
@@ -32,7 +34,8 @@ const DEFAULTS: Settings = {
   downloadArchive: false,
   customCommandEnabled: false,
   defaultTemplateId: null,
-  notifyOnComplete: true
+  notifyOnComplete: true,
+  hasCompletedOnboarding: false
 }
 
 /** Fixed path for the --download-archive file; not user-configurable. */
@@ -104,7 +107,12 @@ export function setSettings(partial: Partial<Settings>): Settings {
     if (value === undefined) continue
     switch (key) {
       case 'theme':
-        if (value === 'light' || value === 'dark') s.set('theme', value)
+        if (value === 'light' || value === 'dark' || value === 'system') s.set('theme', value)
+        break
+      case 'accentColor':
+        if ((ACCENT_COLORS as readonly string[]).includes(value as string)) {
+          s.set('accentColor', value as Settings['accentColor'])
+        }
         break
       case 'defaultKind':
         if (value === 'video' || value === 'audio') s.set('defaultKind', value)
@@ -120,6 +128,7 @@ export function setSettings(partial: Partial<Settings>): Settings {
       case 'downloadArchive':
       case 'customCommandEnabled':
       case 'notifyOnComplete':
+      case 'hasCompletedOnboarding':
         if (typeof value === 'boolean') s.set(key, value)
         break
       case 'defaultTemplateId':