feat(app): share an image into Freegle to start an Offer (iOS) [dark]

[edwh]

What

The iOS half of Share-into-Freegle. Adds an iOS share-sheet extension so a
photo can be shared straight into a new OFFER with the image pre-attached —
the iOS counterpart to the Android ACTION_SEND work in #758.

Stacked on #758. This PR is based on feat/share-into-freegle (Android) and
reuses its pendingSharedImages give-flow path. Merge #758 first (GitHub will
retarget this PR's base to master), or merge them together.

⚠️ DARK / flag-gated — safe to merge, does nothing until enabled

Everything is a no-op unless ENABLE_SHARE_EXTENSION=true. By default the
Xcode project, the main app's entitlements and its signing/export are
byte-for-byte unchanged, so this cannot affect the weekly App Store
release until deliberately enabled. The disabled export composes to exactly
today's app(+NSE) provisioning map.

Enabling requires these human, Apple-Developer-portal steps first (no code
can do them — they need interactive Apple ID + 2FA):

1. Register App Group group.org.ilovefreegle.iphone; enable it on both App
   IDs (org.ilovefreegle.iphone and org.ilovefreegle.iphone.ShareExtension).
2. Pre-create the .ShareExtension App ID + an App Store distribution profile
   (same "create once by hand" approach as the NSE).
3. Regenerate the main app App Store profile so it includes the App Group.
4. Set ENABLE_SHARE_EXTENSION=true in the deploy-apps CircleCI context.

…then on-device verification (native code is not buildable locally).

How

• ShareViewController.swift — no-UI handler: copies the shared image(s) into
  the App Group container, then opens the host app with
  freegleshare://shared?p=<path>… via the responder-chain openURL: technique.
• ShareExtension/Info.plist — com.apple.share-services, image activation
  rule, programmatic principal class (no storyboard to get wrong).
• ShareExtension.entitlements — App Group.
• App/Info.plist — registers the freegleshare:// URL scheme.
• add_share_extension_target.rb — gated; adds the target and (build-time
  only, never committed) the App Group entitlement to the main app. Mirrors
  add_nse_target.rb.
• Fastfile — gated wiring + signing + a composed export map (disabled build
  == today's export).
• stores/mobile.js — appUrlOpen catches freegleshare://shared and queues
  the image(s) into the same pendingSharedImages → /give/mobile/photos
  path as Android.

Risk / testing

• Default build path is provably unchanged (flag off). Validated: edited Fastfile
  and the new Ruby script both pass ruby -c; all plists are well-formed;
  mobile.js passes node --check.
• Native Swift can't be compiled locally — needs the on-device test above.
• The one runtime assumption to verify on-device: Capacitor.convertFileSrc()
  serving the App Group container path to the WKWebView. If it doesn't, the fix
  is to copy app-group→app-tmp in AppDelegate before forwarding the URL.

Part of Share-into-Freegle. Companion: #758 (Android).

🤖 Generated with Claude Code

[edwh]

Superseded by #766, which consolidates the native-app feature PRs (#756 haptics, #757 app-icon shortcuts, #758 share-into-Freegle Android, #759 share-into-Freegle iOS) into a single PR rebased onto current master. Branch kept (not deleted) in case it's needed.