fix(pair): send an iOS QR scan straight to /pair/unsupported

[dschom]

Because

• Firefox iOS cannot finish a pairing started from a native-camera scan, so the "Continue in Firefox" card is only a tap in front of the "pair not supported" roadblock.
• "Continue in Firefox" reads as progress, which makes landing on the roadblock more confusing than going there directly.
• Android is different: updating Firefox there lets the user re-scan the QR with the native camera, so it keeps the card.

This pull request

• Adds a pairing.ios_handoff setting (PAIRING_IOS_HANDOFF), default false, surfaced as config.pairing.iosHandoff.
• planPairingHandoff plans no hand-off for a non-Firefox iOS browser while the setting is off, so /pair falls through to /pair/unsupported with the scanned hash intact.
• Leaves the Android hand-off path untouched.
• Covers the iOS redirect in the Pair/Index and planPairingHandoff unit tests, and adds an iPhone-UA case to pairingUnsupported.spec.ts.
• Clears mocks between tests in the Pair/Index hand-off describe, which sits outside the block that already did.
• Names the new flag in pairingFlowV2iOS.spec.ts and tests/pairing/CLAUDE.md.

Issue that this pull request solves

Closes: FXA-14467

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: packages/fxa-settings/src/lib/pairing/handoff.ts for the gate; packages/fxa-settings/src/pages/Pair/Index/index.tsx for the fall-through to /pair/unsupported.
• Suggested review order: handoff planner → /pair page → config plumbing → tests.
• Risky or complex parts: the /pair mount effect has several ordering-sensitive branches; the iOS case now reaches the existing !isFirefoxDesktop branch rather than returning early.

Screenshots (Optional)

N/A — no component markup changed; iOS now lands on the existing /pair/unsupported screen instead of the existing hand-off card.

Other information (Optional)

• Rollout: the iOS hand-off is preserved, not deleted. Set PAIRING_IOS_HANDOFF=true to re-enable it once Firefox iOS can finish a pairing started from a native-camera scan.
• pairingFlowV2iOS.spec.ts's hand-off delivery reads the CTA off the rendered page, so that spec now needs PAIRING_IOS_HANDOFF=true on the stack. Documented in the spec header and tests/pairing/CLAUDE.md.
• The new pairingUnsupported.spec.ts case cannot pass against a stack serving pre-fix code, so it was verified by the unit tests locally rather than against a running local stack.