feat: optimize static image loading

[h1065153539-create]

Description

Optimizes the existing static campaign imagery for faster loading and keeps the image rendering path on next/image with explicit sizing.

Related Issue

Closes #7

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Changes Made

• Converted the remaining PNG bitmap assets (example and avatar) to WebP while preserving their display dimensions.
• Updated campaign and author images to import the WebP assets through next/image, including explicit sizes values and responsive image styling.
• Added cache-dependency-path: package-lock.json to the Node setup step so npm cache keys are tied to the lockfile.

Screenshots (if applicable)

No UI layout change intended. The campaign image keeps the same intrinsic 642x294 dimensions and the avatar keeps the same 36px rendered size.

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have run npm run lint and fixed all issues
• I have run npm run typecheck with no errors
• I have run npm run build successfully
• I have tested my changes locally

Additional Notes

Verification run locally:

• npm run lint
• npm run typecheck
• npm run build
• Confirmed generated WebP assets: example.webp is 642x294 and 5,424 bytes; avatar.webp is 48x48 and 946 bytes.

No new runtime dependencies were added.

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