feat(settings): gate pairing v2 on the Firefox version per platform

[dschom]

Because

• Pairing v2 switched on from the pairingVersion Firefox reports in fxa_status, which follows a Firefox pref default, so the rollout waited on Firefox rather than on us.
• We want to enable pairing v2 per platform, from a Firefox version of our choosing.
• pairing.ios_handoff answered the same question, whether Firefox iOS is ready for v2, a second way.

This pull request

• Adds pairing.v2_min_version.{ios,android,desktop} (PAIRING_V2_MIN_VERSION_IOS, _ANDROID, _DESKTOP) to the content-server config and serves it to fxa-settings.
• Decides pairing v2 from the Firefox major version in the user agent when the browser's platform has a minimum configured; a platform without one still defers to fxa_status.
• Routes /pair and /connect_another_device through one shared gate, isPairingV2Enabled.
• Replaces pairing.ios_handoff with "an iOS minimum is configured", so setting the iOS minimum also enables handing a QR scanned outside Firefox to the Firefox iOS app.
• Adds unit tests for the gate and wiring tests for both pages.
• Updates the dev-launcher and functional-test pairing notes.

Issue that this pull request solves

Closes: FXA-14582

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/v2-gate.ts holds the whole decision; the pages only call it.
• Suggested review order: configuration.js, then v2-gate.ts with its test, then Pair/Index/index.tsx, ConnectAnotherDevice/index.tsx, and the two hand-off call sites.
• Risky or complex parts: the user-agent version parsing (FxiOS/ on iOS, Firefox/ elsewhere), and the iOS hand-off now following the iOS minimum.

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

• Rollout: with PAIRING_VERSION=2, set PAIRING_V2_MIN_VERSION_<PLATFORM> to the first Firefox major that should get v2 (0 for every version). A platform left unset keeps today's behaviour. PAIRING_IOS_HANDOFF no longer exists and should be dropped from deployment config; PAIRING_V2_MIN_VERSION_IOS takes its place.
• Naming: the ticket spells the keys pairing.v2.minVersion.iOS. The config uses pairing.v2_min_version.ios to match the neighbouring convict keys, and the client sees it as pairing.v2MinVersion.ios.

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