Add SwiftUI upload example; consolidate apps under examples/

[rohit-fastpix]

FastPix iOS Uploads SDK - Documentation PR

Documentation Changes

• New documentation added
• Existing documentation updated
• Documentation errors fixed
• Code examples updated
• Links and references updated

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Files Modified

• README.md
• docs/ directory
• USAGE.md
• CONTRIBUTING.md
• Other: new SwiftUI example app + examples/ reorg (examples/README.md,
  examples/swiftui/**, examples/BACKGROUND-UPLOADS-SDK-REQUEST.md), UIKit
  example moved to examples/uikit/

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Summary

Brief description of changes:

Adds a SwiftUI example app demonstrating the iOS Uploads SDK, and consolidates
the example apps under a single examples/ folder (one folder per project).

• New examples/swiftui/ — a SwiftUI app that mints a signed upload URL
  in-app (create-upload), picks a video via PhotosPicker, and drives chunked
  upload with live progress and pause / resume / cancel through an
  ObservableObject. References the SDK locally so it builds against this repo.
• Background uploads — wraps the active upload in beginBackgroundTask
  (best-effort). True background continuation needs a background URLSession,
  which is an SDK change; documented in BACKGROUND-UPLOADS-SDK-REQUEST.md.
• Reorg — the existing UIKit example moved from example/ to
  examples/uikit/ (history preserved); added a minimal examples/README.md.
• Credentials are read from a gitignored Secrets.swift (template committed as
  Secrets.example.swift); no keys are committed.

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Code Examples

import fp_swift_upload_sdk

// Keep a strong reference for the whole upload session.
let uploader = Uploads()
uploader.delegate = self          // UploadsDelegate — progress + lifecycle events

// Mint a signed upload URL, then start the chunked upload.
Task {
    let signed = try await FastPixAPI.createUpload()   // POST /v1/on-demand/upload
    uploader.uploadFile(file: fileURL, endpoint: signed.url)
}

// Manage upload lifecycle.
uploader.pause()   // Pause upload
uploader.resume()  // Resume upload
uploader.abort()   // Abort upload

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Testing

• All code examples tested on iOS
• Links verified
• Grammar checked
• Formatting consistent

Notes: SwiftUI example builds clean against the local SDK (iOS 16+
simulator); create-upload verified against the live API (HTTP 201, returns
data.url + data.uploadId).

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Review Checklist

• Content is accurate
• Code examples work as expected
• Links are working
• Grammar is correct
• Formatting is consistent

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Ready for review!