Add app logo and platform icons

[Copilot]

Implements logo and icon assets for the Improv Assistant app.

Design

• Comedy mask with indigo background (#3F51B5, matches Material theme)
• Gold microphone accent
• 512×512 base resolution, programmatically generated via PIL

Assets

Android (16 files)

• Launcher icons: mdpi through xxxhdpi densities
• Round variants for circular displays
• Adaptive icon foregrounds (Android 8.0+)
• Updated background colors: ic_launcher_background.xml from #008577 → #3F51B5

iOS (15 files)

• All required sizes (20×20 through 1024×1024)
• Scale factors @1x, @2x, @3x
• App Store marketing icon

Documentation

• docs/LOGO.md: design specs, color palette, regeneration guidance
• docs/assets/logo.png: 512×512 preview

Original prompt

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This section details on the original issue you should resolve

<issue_title>create logo / icon</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes create logo / icon #8

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Summary by cubic

Adds the project logo and native app icons for Android and iOS so the app shows the new branding on devices, and sets the Android adaptive icon background to the indigo theme color (#3F51B5). Implements issue #8 (create logo / icon).

• New Features
  • Added Android launcher, round, and adaptive foreground icons for all densities.
  • Updated iOS AppIcon.appiconset with required sizes up to 1024x1024.
  • Included ic_launcher-web.png for Flutter web and added docs/LOGO.md with logo assets.

^{Written for commit 230bc78. Summary will update on new commits.}