Add hero screenshot/GIF to README

[Pivii]

Goal

Add a visual hero (screenshot or short GIF) at the top of the README, right under the badges and nav row. This is the single highest-impact change to convert a GitHub visitor into a star.

Why

The README has screenshots further down (Keyboard / Model Manager / Settings) but nothing visual above the fold. Pulling a single highlight to the top significantly improves star conversion.

Acceptance criteria

• A docs/hero.gif (or .png / .webp) in the repo, ~600–900px wide, <3 MB
• Shows the keyboard in action: open an app → switch to Dictus keyboard → speak → text appears
• Embedded in README right after the nav row, before the "Why Dictus?" section
• Renders cleanly on both light and dark GitHub themes
• Includes meaningful alt text

Suggested content

3–5 seconds, looped:

1. Open WhatsApp / Messages, focus in the text field
2. Switch to Dictus from the keyboard switcher
3. Tap mic, dictate a short sentence (FR or EN)
4. Text appears in the message field

How to record

• Android screen recording (Quick Settings → Screen Record), or adb shell screenrecord /sdcard/hero.mp4
• Convert: ffmpeg -i hero.mp4 -vf "fps=15,scale=720:-1" -loop 0 hero.gif
• Or .webp: ffmpeg -i hero.mp4 -vf "fps=15,scale=720:-1" -loop 0 hero.webp

Out of scope

• Open Graph image (tracked separately)