[fix] Keep Flow Chat file change counts session-scoped

[limityan]

Summary

• Keep Flow Chat header and file bar counts scoped to the current chat session.
• Derive session file stats from completed per-session snapshot boundaries instead of global baselines versus the current workspace, so rebases or later edits do not inflate old session counts.
• Drop stale cached or in-flight file stats when the active session file list shrinks.

Product Semantics

• A chat file count represents completed file changes made by that chat session only.
• Failed or unfinished file operations are not counted.
• Files whose net session diff is empty remain hidden from the count.
• Review and Deep Review launch actions use the same session-scoped visible file set.

Validation

• cargo test -p bitfun-core snapshot_core::tests -- --nocapture
• cargo test -p bitfun-core
• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run
• cargo check --workspace (passes with existing bitfun-cli unused import warning)
• cargo test --workspace (passes with existing bitfun-cli unused import warning)