ux: make Inventory + Admin sub-tabs URL-addressable with default-first (QA A.4)

[cristim]

QA A.4: Inconsistent sub-tab restoration

The Inventory & Coverage page and the Admin page handle sub-tab state inconsistently:

• Inventory & Coverage remembers the last visited sub-tab within a session (PR fix(inventory): default sub-tab is Active Commitments, not RI Exchange #757 made it default to Active Commitments on first load, but it persists the last sub-tab via hidden session state).
• Admin always resets to its General sub-tab on every entry.

This inconsistency is the actual defect.

Chosen approach (best practice)

Make sub-tabs URL-addressable and default-first, applied identically to BOTH pages:

• Sub-tab selection reflected in the URL, matching the existing topbar global-filter convention (?provider=&account=).
• Fresh navigation with no sub-tab param lands on the FIRST/default sub-tab (Inventory -> Active Commitments; Admin -> General).
• Switching a sub-tab updates the URL without a full reload (shareable/bookmarkable, back/forward works).
• Re-entering with a sub-tab param lands on that sub-tab (preserves the "remembering" benefit without hidden session state).
• Preserve any existing deep links.

This supersedes the partial #757 behavior (session-memory) with explicit, shareable URL state.