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

[cristim]

Summary

Resolves QA A.4: Inventory & Coverage and Admin handled sub-tab state inconsistently.

• Inventory & Coverage remembered its last sub-tab via hidden in-memory session state (the partial behavior from fix(inventory): default sub-tab is Active Commitments, not RI Exchange #757).
• Admin always read its sub-tab from the /admin/<subtab> URL.

That inconsistency was the defect. This PR makes Inventory sub-tabs URL-addressable as /inventory/<subtab>, matching the existing Admin convention, so both pages behave identically and the sub-tab is shareable/bookmarkable with working browser back/forward.

Changes

• loadInventory() derives the sub-section from the URL path (getInventorySubTabFromPath), not the module-level currentSubSection. Fresh /inventory lands on the default (active-commitments); a /inventory/<subtab> deep link lands on that sub-tab.
• Sub-nav clicks route through navigation.switchInventorySubTab, which pushes /inventory/<subtab> via history.pushState (preserving existing query params + hash, no full reload, no duplicate entry on a no-op switch).
• The history push lives in navigation.ts alongside switchSettingsSubTab so the single historyId counter stays authoritative for the back/forward dirty-guard; inventory.ts keeps the pure DOM view switch.

Supersedes the partial #757 session-memory behavior with explicit URL state. Existing /admin/* deep links are unaffected.

URL scheme

Path-based /inventory/<subtab> and /admin/<subtab>. This matches the app's established sub-tab convention (Admin already used /admin/<subtab> with full pushState/popstate handling in navigation.ts). The topbar global filter uses query params (?provider=&account=); that convention is for orthogonal cross-section filter state, not sub-tab identity, and is preserved across sub-tab switches.

Tests

For each page: no param -> default/first sub-tab; explicit param -> that sub-tab; switching updates the URL (query/hash preserved); unknown param -> default. Full frontend suite green (2177 pass).

Closes #902.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ff280587-9b62-49fc-9cca-1ed956e1c8dc

📥 Commits

Reviewing files that changed from the base of the PR and between 9476456 and 21cf13f.

📒 Files selected for processing (4)

• frontend/src/__tests__/inventory.test.ts
• frontend/src/__tests__/navigation.test.ts
• frontend/src/inventory.ts
• frontend/src/navigation.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/url-addressable-subtabs

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