feat(visualization): add collapsible mode to File Explorer sidebar

Adds an in-memory collapse toggle for the left-side File Explorer drawer:

- A new `«` button at the right end of the EXPLORER header collapses the
  sidebar to a small floating row pinned at the top-left, freeing the
  codemap area. The collapsed widget contains a folder-tree expand
  button plus the existing search input, clear button, and kebab menu
  (Flatten / Exclude). Re-expand returns to the full layout.
- State lives in a root-provided signal-backed `ExplorerCollapseService`;
  no persistence — the sidebar opens expanded on every reload.
- The search-bar kebab is migrated from DaisyUI's `dropdown` (clipped by
  the host's `overflow-hidden` in collapsed mode) to the native
  `[popover]` API + CSS anchor positioning, matching the pattern used by
  the FLATTENED / HIDDEN rule popovers. The menu now renders in the
  browser's top-layer and works in both modes.
- Feature components stay zoneless-compatible (signals only, OnPush, no
  setTimeout/NgZone/CDR).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>