fix(search): preserve list search focus across rebuilds

[CharlVS]

Summary

• preserve search focus and cursor state across rebuilds in wallet assets, coins manager, bridge ticker list, DEX coin filters, and NFT desktop filters
• move controller and focus ownership to stable parents so filtered-list updates do not recreate editing state
• keep existing debounce timings, filter behavior, and widget keys unchanged

Validation

• reviewed lifecycle and bloc sync paths for focus regressions and feedback loops
• ran dart format on the touched files
• ran targeted flutter analyze on the changed files with no issues found
• did not run unit or integration tests because repository guidance states they are currently failing globally; validated through static analysis and code review

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Visit the preview URL for this PR (updated for commit b8f81ba):

https://walletrc--pull-3451-merge-d8rrq9xv.web.app

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