docs(brand): replace icon set with 3-tone fan mark

[ajianaz]

What

Replaces the Cosy icon set (favicon, PWA icons, apple-touch) with renders of the approved 3-tone Fan mark — previously the only committed icons were placeholder solids. Adds them under docs/public/logo/ alongside the symbol variants from #125.

Why

The 3-tone mark (blue #1864AB / cyan #22B8CF / orange #E8590C) is the approved brand treatment; icon surfaces should carry it, not a flat placeholder. Small sizes use the dedicated small-size cut (filled 3-tone with white separators) that survives 16 px.

How

• favicon.ico (16/32/48) + favicon.svg + favicon-{16,32,48}.png — from the small-size cut
• icon-{192,512}.png — master mark, transparent (PWA any-mask)
• maskable-512.png — full-bleed warm-white with 22% safe-zone padding (opaque, mask-safe)
• apple-touch-icon.png — 180 px on white
• README table extended with the new files

Testing

• Vision QA (GLM-4.6V) on all 5 icon shapes: 3-tone order correct everywhere, 16 px still reads as the fan, no glitch
• maskable verified opaque + safe-zone padded (no clipping under circular masks)
• Asset-only diff — no code paths touched

Related Issues

• Follow-up to docs(brand): add cosy logo symbol variants #125

Checklist

• Branch name follows convention (docs/)
• Branch is from develop
• Commit messages follow Conventional Commits
• No secrets or credentials committed
• One logical change per PR (no mixed concerns)

[github-actions]

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✅ No issues found. Code looks good!

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