feat(watchos): Apple Watch app + BTC price complication

[islandbitcoin]

What

Adds a native watchOS app and a WidgetKit complication to Flash that show a live BTC price on the watch and on any watch face. Built to mirror the conventions of the existing home-screen widget (ios/FlashWidget).

Linear: ENG-459 — https://linear.app/island-bitcoin/issue/ENG-459

Why

Glanceability. Users get the BTC price (in their display currency) on their wrist and watch face without opening the phone app — even when the phone is away.

How it works

• Self-contained price fetch — the watch app and complication call the public, unauthenticated realtimePrice GraphQL query (https://api.flashapp.me/graphql), the same one the home-screen widget uses. Works before login; defaults to USD.
• Watch-local App Group group.com.lnflash.watch shares the price snapshot between the watch app and its complication. (The iOS group.com.lnflash lives on a different device and can't be reused.)
• WatchConnectivity carries the user's display currency phone → watch so the watch reprices in the chosen currency. Best-effort; the watch works standalone without it.
• Complication uses WidgetKit accessory families (watchOS 9+): circular, inline, rectangular, corner. ClockKit (deprecated) is not used.
• Quick actions — Scan/Receive in the watch app send a message to the phone, which opens flash://scan / flash://receive via the existing React Navigation linking (PREFIX_LINKING already includes flash://).

Files

Area	Files
Watch app	ios/FlashWatch Watch App/ — FlashWatchApp, ContentView, PriceService, WatchStore, PhoneConnectivity
Complication	ios/FlashWatchComplication/ — bundle, provider, accessory views
Phone bridge	ios/LNFlash/WatchConnectivityBridge.{swift,m}
JS glue	app/utils/watch.ts, app/components/watch-currency-sync/ (mounted in app/graphql/client.tsx)
Docs	ios/FlashWatch/SETUP.md

Reviewer / merge note

Creating the two watch Xcode targets edits project.pbxproj, which (per the widget's ios/FlashWidget/SETUP.md) is done in the Xcode GUI to avoid corrupting the project file. All Swift sources, Info.plists, and entitlements are scaffolded here; ios/FlashWatch/SETUP.md has the exact step-by-step (target creation, target membership, App Group, deployment target, build/run + complication install). The JS side is wired and active immediately and no-ops on Android / when no watch is paired.

Testing

• TypeScript: the JS mirrors the proven widget-price-sync pattern; useDisplayCurrency() returns the exact fractionDigits / fiatSymbol / displayCurrency fields consumed. (Repo-wide tsc:check currently errors with a pre-existing TS5095 config issue on a clean main too — unrelated to this change.)
• Native build is verified manually via SETUP.md (CI here has Command Line Tools only, no xcodebuild).

🤖 Generated with Claude Code

[linear]

ENG-459