fix(ui): redirect legacy slack workspace route

[onutc]

Summary

• redirect legacy /spritz/slack-gateway/* routes to the real server-rendered gateway paths
• preserve query and hash fragments during the redirect
• add regression coverage for the URL mapping and redirect behavior

Validation

• pnpm --dir ui test -- --run App.test.tsx
• pnpm --dir ui typecheck
• staged manual rollout on staging: rebuilt spritz-ui and spritz-slack-gateway, verified the blank page was replaced by the auth redirect flow instead of an empty route shell

[gitrank-connector]

👍 GitRank PR Analysis

Score: 20 points

Metric	Value
Component	Other (1× multiplier)
Severity	P2 - Medium (20 base pts)
Final Score	20 × 1 = 20

Eligibility Checks

Check	Status
Issue/Bug Fix	✅
Fix Implementation	✅
PR Documented	✅
Tests	✅
Lines Within Limit	✅

Impact Summary

This PR fixes a routing bug where users accessing legacy /spritz/slack-gateway/* paths would encounter blank pages. The fix implements proper client-side redirects to the real server-rendered gateway paths while preserving query parameters and hash fragments. The changes include refactoring hardcoded auth header constants into configurable values, adding comprehensive test coverage for both the redirect behavior and custom header configuration.

Analysis Details

Component Classification: This PR spans both backend (Go) and frontend (React/TypeScript) changes across the Slack gateway integration and UI routing. Since it doesn't fit neatly into a single specialized component category and involves cross-cutting concerns, OTHER is the appropriate classification.

Severity Justification: This is a P2 (Medium) severity fix addressing a functional bug where legacy SPA-mounted Slack gateway routes were rendering blank pages instead of redirecting to the real server-rendered gateway. The issue has a workaround (direct navigation) and doesn't cause data loss or service outages, but it does impact user experience and breaks a documented route pattern.

Eligibility Notes: Issue: True - PR clearly fixes a broken routing behavior (blank page instead of proper redirect). Fix Implementation: True - code changes align with the stated goal of redirecting legacy routes. PR Linked: True - detailed summary with validation steps and manual testing on staging. Tests: True - includes both unit tests (App.test.tsx) and integration tests (gateway_test.go). Tests Required: True - this involves business logic changes (routing behavior) and API changes (making auth headers configurable), both of which require test coverage to prevent regressions.

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