fix(web-ui): keep FlowChat collapses left aligned - #1842

Summary

• remove leading indentation from FlowChat collapse projection roots and expanded content across thinking, explore, task/subagent, terminal, Git, MiniApp, todo, image, and shared tool cards
• add a message-list guardrail that keeps collapsible rows left aligned without flattening ordinary card-internal spacing
• document the alignment invariant and add regression coverage for all affected styles

Testing

• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/FlowChatCollapseAlignment.test.ts src/flow_chat/components/modern/SmoothHeightCollapse.test.tsx src/flow_chat/tool-cards/TaskToolDisplay.test.tsx src/flow_chat/tool-cards/TodoWriteDisplay.test.tsx src/flow_chat/tool-cards/GitToolDisplay.test.tsx src/flow_chat/components/modern/SubagentProjectionView.test.tsx
• compiled all touched SCSS entry points successfully