fix(tab-navigation): overflow menu for route-backed tabs without ids

[roncodes]

Summary

TabNavigation moves tabs that do not fit into a "More" dropdown, but the overflow bookkeeping keyed everything on tab.id. Route-backed tabs (the common shape in Fleet-Ops detail panels: { route, label }) have no id, so:

• visibleTabIds and overflowTabIds were arrays of undefined, which made both the visible filter and the overflow filter match every tab;
• the tab list rendered all tabs inline (clipped by .tab-items { overflow: hidden }), so trailing tabs were unreachable and the More menu never appeared.

This change derives a tab's identity from id, then route, then key, and uses it consistently for the active-tab state, overflow calculation and keyboard navigation. Tabs that already pass id are unaffected.

Tests

• Added an integration test covering route-only tabs overflowing into the More menu (the dummy router gains three routes as link targets).
• eslint and prettier pass on the touched files.
• The browser test suite was not run locally for this change (no headless browser session was started on the development machine); CI should run it.