feat: multi-account header, signing & balance support

[BitHighlander]

Summary

• Header refactor: Split monolithic NetworkAccountHeader (837 lines) into focused components — NetworkDropdown, AccountDropdown, AddNetworkModal, and shared utilities
• ETH multi-account: Show all ETH-derived accounts on every EVM chain (Polygon, Base, Arbitrum, etc.), not just Ethereum mainnet. Add Account button works from any EVM network view
• Signing fix: Replace hardcoded m/44'/60'/0'/0/0 derivation path with dynamic lookup based on selected account. Update global ADDRESS on account switch so eth_accounts, personal_sign, eth_sendTransaction, and eth_signTypedData_v4 all use the correct account
• Balance filtering: AssetDetail now shows per-account balance instead of combined total across all accounts
• Sticky header: Header stays visible above drawers (Send/Receive/AssetDetail) with proper z-index layering
• UX improvements: Chainlist.org link, interactive address copy, derivation path display, scrolling fixes

Key files changed

File	Change
ethereumHandler.ts	getAddressNListForAddress() helper, dynamic path in signTransaction/signTypedData/signMessage
background/index.ts	Update ADDRESS on SET_ASSET_CONTEXT for EVM accounts
header/headerUtils.ts	buildNetworkList() and buildAccountList() — ETH accounts visible on all EVM chains
header/NetworkDropdown.tsx	Network selector grouped by chain family
header/AccountDropdown.tsx	Account selector with derivation paths, add/remove
AssetDetail.tsx	Filter balances by selected account address
SidePanel.tsx	Sticky header, drawer height calc, asset context lifecycle

Test plan

• Open side panel, verify header shows KeepKey branding on dashboard
• Click a balance row — asset drawer opens below header, header stays visible
• Network dropdown shows chains grouped by family (EVM, UTXO, Cosmos, Other)
• Account dropdown on Ethereum shows Account 0, Account 1, etc. with derivation paths
• Account dropdown on Polygon/Base/Arbitrum also shows all ETH accounts (not just account 0)
• Add Account button works from any EVM chain view
• Switching accounts updates the displayed balance to that account only
• Sign a transaction from Account 1 — verify device signs with correct derivation path
• personal_sign from Account 1 — verify correct address used
• Close asset drawer — header returns to branding mode

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