fix: Currency API fetch works in Web Worker context (fixes #18)

[konard]

Summary

This PR fixes the root cause of issue #18 where exchange rates were showing as "hardcoded" with "unknown" date instead of using real-time API data.

Root Cause Analysis

The currency exchange rate API fetch was silently failing because web_sys::window() returns None in Web Worker contexts. Web Workers don't have a window object - they use WorkerGlobalScope instead.

Before (broken):

let window = web_sys::window()
    .ok_or_else(|| CurrencyApiError::NetworkError("No window object available"))?;
// Always fails in Web Worker!

After (fixed):

let global = js_sys::global();
if let Some(window) = global.dyn_ref::<web_sys::Window>() {
    // Window context
} else if let Some(worker) = global.dyn_ref::<web_sys::WorkerGlobalScope>() {
    // Web Worker context
}

Changes

1. src/currency_api.rs: Use js_sys::global() to detect and use either Window or WorkerGlobalScope context for fetch operations
2. Cargo.toml: Add WorkerGlobalScope feature to web-sys dependency
3. docs/case-studies/issue-18/README.md: Update documentation with root cause analysis
4. changelog.d/: Add changelog fragment for the fix

Test Plan

• cargo fmt --check passes
• cargo clippy passes
• cargo test passes (all 66+ tests)
• Currency API fetch should now work in the Web Worker
• Live site shows real exchange rates from API (pending CI/deploy)

Related

• Fixes Проблема с выражением: 0 RUB + 1 USD #18
• Previous fix attempt in PR fix: Apply exchange rates from API to Calculator (fixes #18) #38 added rate application code but API fetch was never working

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[konard]

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