fix(icon): stabilize fallback rendering (mount flash, layout shift, spinner)

[JeanMarcMilletScality]

Split out of #1120, which mixed unrelated table, icon, and storybook changes. This PR contains only the Icon work.

Changes

• Seed cached icon synchronously to avoid a mount flash
• Reserve the icon box in the delayed fallback to avoid layout shift
• Scale the fallback spinner with size instead of a fixed px value
• Keep the fallback spinner purple (Loader default color)
• Add stories for fallback box reservation and loader-in-box

Files

• src/lib/components/icon/Icon.component.tsx
• stories/icon.stories.tsx

Verification

npm run build-storybook completes successfully (exit 0).

Note: this branch's own build-storybook check will fail until #1121 (guideline RadioGroup import fix) merges into development/1.0 — that break is inherited from the base branch, not introduced here. Verified locally by building with #1121's fix applied.

🤖 Generated with Claude Code

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