fix(web): polish session search and ordering

[oodavy41]

Summary

• align the session list search icon and custom clear button with the input center
• hide the browser-native search cancel button to avoid duplicate clear buttons
• keep collapsed session previews ordered by activity instead of promoting the selected session
• auto-expand the selected session's preview group so deep-linked sessions remain visible

Testing

• bun --cwd web typecheck
• bun --cwd web test --run src/components/SessionList.test.ts

[github-actions]

Findings

• [Major] Selected sessions past the preview limit can remain hidden — isSessionGroupExpanded treats a missing sessions::... override as collapsed, and a false override as expanded. The new effect deletes a collapsed preview override instead of setting the expanded state, so after removing selected-session promotion a deep-linked selected session beyond the first 8 rows still is not rendered. Evidence web/src/components/SessionList.tsx:796.
  Suggested fix:

  const isPreviewExpanded = prev.get(sessionPreviewKey) === false
  if (!isPreviewExpanded) {
      next.set(sessionPreviewKey, false)
      changed = true
  }

Summary
Review mode: initial
One issue found: selected-session auto-expansion does not actually expand collapsed session previews because this collapse key uses inverted boolean semantics. Residual testing gap: no component/helper coverage for a selected session beyond GROUP_SESSION_PREVIEW_LIMIT becoming visible.

Testing
Not run (automation; review-only and PR code treated as untrusted).

HAPI Bot

[github-actions]

[MAJOR] Selected sessions past the preview limit can remain hidden. isSessionGroupExpanded treats a missing sessions::... override as collapsed, and a false override as expanded; deleting the key here leaves the default collapsed state in place, so a deep-linked selected session beyond the first 8 rows still is not rendered after selected-session promotion was removed.

Suggested fix:

const isPreviewExpanded = prev.get(sessionPreviewKey) === false
if (!isPreviewExpanded) {
    next.set(sessionPreviewKey, false)
    changed = true
}