PROJ-670: ProjectNav Priority+ More menu for mobile tab overflow

[TAJD]

Summary

• Replaces the mobile horizontal-scroll tab bar in ProjectNav with a Priority+ pattern: as many leading tabs are shown as fit the available width, with the rest collapsed into a "More ▾" menu.
• Measures each tab's natural width once (labels are static) and the nav container's width via ResizeObserver (falls back to showing all tabs if measurement is unavailable, e.g. containerWidth is 0 — this also keeps existing/unstubbed tests passing unchanged).
• If the active tab has overflowed into the menu, the "More" trigger gets aria-current="true" and active styling so the current section is still indicated.
• Menu items are ≥44px tall tap targets, the trigger exposes aria-haspopup="menu"/aria-expanded, and the menu closes on selection, outside click, and Escape (focus returns to the trigger).
• Removed the old max-sm:overflow-x-auto horizontal scroll class — no more horizontal scrolling of the tab row at any width.

Design decisions

• Widths are measured once on mount (tab labels are static/fixed) rather than continuously, avoiding a remeasure-after-hide correctness bug (a display:none element reports 0 width in real browsers).
• The "More" trigger's own width is a fixed reserve constant rather than measured, since it only ever renders once other tabs are already hidden (avoiding a chicken-and-egg measurement problem).
• Tests stub clientWidth/offsetWidth on HTMLElement.prototype (jsdom doesn't compute real layout) to exercise the overflow math deterministically — see stubNavMeasurements in ProjectNav.test.tsx.

Test plan

• pnpm --filter @projektor/web test — all 680 tests pass (15 in ProjectNav.test.tsx, including 5 new Priority+ overflow tests)
• pnpm --filter @projektor/web test:coverage — passes
• pnpm turbo type-check --filter=@projektor/web — 0 errors
• pnpm biome check on changed files — clean
• pnpm --filter @projektor/web build — succeeds
• Manual check at iPhone-width viewport — not performed this session (browser automation tooling unavailable/unauthenticated); relying on the automated viewport/overflow-simulation tests instead.

Closes PROJ-670.

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