fix(web): persist file explorer expanded tree and scroll position across navigation

[swear01]

Summary

• Expanded folder state in the Directories tab was stored in local React state; navigating to a file and back caused DirectoryTree to unmount and reset to only the root node
• Scroll position of the file list was similarly lost on remount
• Both are now persisted in sessionStorage keyed by sessionId and restored on remount

Changes

• DirectoryTree.tsx: reads initial expanded from sessionStorage on mount; writes on every toggle via useEffect
• files.tsx: adds a ref on the scroll container; restores scrollTop from sessionStorage on mount and saves it on unmount

Test plan

• Open Files → Directories tab for a session
• Expand several nested folders and scroll down
• Click a file to view it, then press back
• Confirm the tree is still expanded and scroll position is restored
• Confirm that a fresh session (no prior sessionStorage entry) still opens with only the root expanded

Fixes #910

🤖 Generated with Claude Code

[github-actions]

Findings

• [Major] Reset persisted tree state when the session id changes — useState(() => readExpanded(props.sessionId)) only reads storage on the first mount, but this route can be reused while $sessionId changes (web/src/router.tsx:992). The next render keeps the previous session's expanded set and the new write effect stores it under the new session key, so opening another session's Files page can inherit/overwrite the wrong tree state. Evidence: web/src/components/SessionFiles/DirectoryTree.tsx:203.
  Suggested fix:

  <DirectoryTree
      key={sessionId}
      api={api}
      sessionId={sessionId}
      rootLabel={rootLabel}
      onOpenFile={(path) => handleOpenFile(path)}
  />

Summary

• Review mode: initial
• One state scoping issue found in the new sessionStorage-backed directory expansion persistence.

Testing

• Not run (automation): bun typecheck unavailable in this runner (bun: command not found).

HAPI Bot

[github-actions]

[Major] useState(() => readExpanded(props.sessionId)) only reads storage on the first mount. Because the files route is under the parameterized $sessionId route, navigating from one session's Files page to another can reuse this component with a new sessionId; the old expanded set is then written under the new session key by the effect below. Key the tree by sessionId (or otherwise reset state before writing for a new id) so each session hydrates its own persisted state.

Suggested fix:

<DirectoryTree
    key={sessionId}
    api={api}
    sessionId={sessionId}
    rootLabel={rootLabel}
    onOpenFile={(path) => handleOpenFile(path)}
/>

[github-actions]

Findings
No issues found.

Questions
None.

Summary

• Review mode: follow-up after new commits
• Previous HAPI Bot finding is addressed by remounting DirectoryTree per sessionId at web/src/routes/sessions/files.tsx:467.
• Residual risk: no automated coverage found for the new directory expansion and scroll restoration behavior; current validation appears manual.

Testing

• Not run (automation)

HAPI Bot