fix(wallet): Fee icon on Cow Swap confirmation screen is blank on iOS

[StephenHeaps]

• Aligned with WebUI to display placeholder network icon using chainName for network fee on CoW Swap safer sign UI.
• Fixed Blockie background implementation (renamed BlockieMaterial to BlockieBackground for clarity)
  • WebUI / desktop is using backgroundColor & spotColor (color 2 & color 3 from generator) for Blockies.background, where bgColor & spotColor are 2nd / 3rd color respectively
  • iOS was using color & backgroundColor (color 1 & color 2 from generator):

    brave-core/ios/brave-ios/Sources/BraveWallet/Blockies/Blockies.swift

    Lines 162 to 168 in d04b4ad

    	var body: some View {
    	LinearGradient(
    	colors: [.init(blockies.makeColor()), .init(blockies.makeColor())],
    	startPoint: .top,
    	endPoint: .bottom
    	)
    	}

• Aligned Blockie seed usage with WebUI
  • Addresses are always lowercased when used as a seed (matches existing behaviour):

    brave-core/components/brave_wallet_ui/common/hooks/use-orb.ts

    Line 39 in 02e3dc0

    accountInfo.address?.toLowerCase() ||

  • Asset name (used when contract address is empty) not lowercased:

    brave-core/components/brave_wallet_ui/components/shared/create-placeholder-icon/index.tsx

    Lines 119 to 121 in 3ee579e

    	seed: asset?.contractAddress
    	? asset?.contractAddress.toLowerCase()
    	: asset?.name

  • Network (chain name) not lowercased:

    brave-core/components/brave_wallet_ui/common/hooks/use-orb.ts

    Line 87 in 02e3dc0

    seed: networkInfo.chainName

  • Sha256 (Bitcoin seed) not lowercased:

    brave-core/components/brave_wallet_ui/common/hooks/use-orb.ts

    Lines 41 to 43 in 02e3dc0

    	.createHash('sha256')
    	.update(accountInfo.accountId.uniqueKey)
    	.digest('hex')

  • This change affects Asset Icon placeholder gradient, Network Icon placeholder gradient, and Account Card backgrounds. This aligns our colours with desktop for a uniform style across platforms.

Resolves brave/brave-browser#36697

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Submit a swap transaction on swap.cow.fi using Gnosis network (important to use network not preloaded / network without an icon)
2. Check the panel and verify network icon is not an empty circle.

iOS	Desktop (for compare)