chore(branding): replace Claude-style icon with Heicode H mark

User-supplied screenshot of the Heicode H circuit logo (heicode-logo.png
at repo root) was background-removed via flood-fill from the image
borders, edge-feathered with a 0.7px Gaussian on the alpha channel, and
upscaled to 1024x1024 as the master.

Replaced everywhere the icon is referenced:
  src-tauri/icons/
    32x32.png, 128x128.png, 128x128@2x.png  — Tauri build inputs
    icon.ico  — multi-res 16/24/32/48/64/128/256 (Windows installer +
                taskbar)
    icon.icns — multi-res 16/32/64/128/256/512/1024 (macOS bundle)
    Square*.png + StoreLogo.png — Windows store sizes (kept in sync)
  public/app-icon.png — splash icon shown by HeicodeLoginPage,
                        ActiveSession.tsx, EmptySession.tsx,
                        Settings.tsx (1024x1024)

The H mark sits on transparent alpha now; on dark window chrome it
appears as the floating logo without a white card. Source resolution
(273x276) means 16/24px renderings are slightly soft; adequate for
taskbar/tray and crisp at 32px+.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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