Feat/cost attribution 868

[Robinsonchiziterem]

Summary

Implements cost attribution by application tag or memo prefix for project budgeting (#868). Attributes transaction fees (in Stroops and XLM) and payment transfer volumes to developer-defined project tags (e.g. billing, auth, nft-drop, defi-swap) via literal memo prefixes (e.g., [APP:billing], BILL:) or custom regex patterns.

• Domain Library (src/lib/costThresholdManager.ts): CostThresholdManager implementation for tag management, memo prefix and regex pattern matching, Stroops/XLM fee calculation, volume tracking, threshold alerts (ok, warning >= 80%, exceeded >= 100%, unbudgeted), auto-discovery of embedded tags ([APP:tag]), JSON/CSV report exports, and graceful SSR/storage error handling.
• Domain Library (src/lib/costThresholdManager.ts): CostThresholdManager implementation for tag definition, memo prefix and regex pattern matching, budget limits, volume tracking, threshold alerts (ok, warning >= 80%, exceeded >= 100%, unbudgeted), auto-discovery of embedded tags ([APP:tag]), JSON/CSV report exports, and graceful SSR/storage error handling.
• Analytics Integration (src/lib/analytics.ts): Embedded cost attribution calculations into buildAnalyticsSnapshot() and re-exported CostThresholdManager.
• Documentation: User-facing & developer guide in docs/features/COST_ATTRIBUTION_ANALYTICS.md and changelog entry in CHANGELOG.md.
• Tests were added or updated for the change (primary flow, a boundary case, and a failure case).
• Docs were updated where behaviour, configuration, or security posture changed.

Closes #868

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[Manuelshub]

@Robinsonchiziterem Please resolve conflicts.