Make Tab Groups Save-able to Collections

[mbloflin]

Problem: Tab Groups are great in-session but vanish on close; Collections save tabs but flatten them, losing context. Today, you cannot drag a Group into Collections, there’s no “Add Group,” and even multi‑select adds only the last tab—leading to tedious one‑by‑one saving.
Feature Request:

Add “Save Tab Group to Collection” in the Tab Group menu. Save the Group as‑is (name + tabs). Clicking it later should reopen all Group tabs.
Fallback: If full grouping is heavy, keep “save tabs” but auto‑insert a section header based on the Group name to preserve context.

Benefits: Maintains structure, avoids dumping‑ground Collections, enables fast recall for research/news/projects, and leverages existing Collection sections for a lightweight implementation.
Repro (current behavior):

Right‑click Group → no option to add to Collections.
Collections pane: cannot drag the Group.
Shift‑select tabs → “Add page” adds only the last selected tab.