Build: Web UI v2 — bulk operations + trash + export

[paperhurts]

Follow-up to #24. Spec'd in spec.md §'Web UI'.

Scope

• Bulk select + tag/move/delete operations on the listing.
• Trash view showing soft-deleted bookmarks (deleted_at != null, still within the 30-day GC window), with restore action.
• Netscape HTML export for emergency portability.

Acceptance

• Multi-select UX in the listing.
• Bulk apply tag / change folder / soft-delete → single batched client.update() call (one commit per bulk action).
• Trash listing filters deleted_at != null; restore clears deleted_at.
• Export downloads a valid Netscape HTML bookmarks file.