test(web): make browser harness cross-engine faithful

[0xkkonrad]

Why

The nightly cross-browser matrix exposed deterministic harness assumptions: programmatic focus was treated as keyboard focus, the link-share test requested Chromium-only clipboard permissions in Firefox, and the mobile project combined an iPhone/WebKit device descriptor with Firefox. A cold WebKit run also showed that typing could race React hydration.

What

• drive drawer focus assertions through the actual keyboard tab order
• stub the clipboard write contract before navigation and assert the copied room payload
• wait for React to own an SSR control before typing, without a timing sleep
• retain the 390px/touch test surface in Firefox using its native UA and supported context capabilities

No product code or product assertion is changed.

Proof

• Chromium desktop focused journeys: 2/2
• Firefox desktop focused journeys: 2/2
• WebKit desktop focused journeys: 2/2
• Firefox narrow-device smoke: focus and share journeys pass; viewport resize completes
• Firefox then exposes a genuine remaining import-fold assertion at 320x700, so this PR remains draft pending that separate product decision
• web typecheck at c471c75: green
• current config plan, Prettier and diff check: green

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