Expand gradient troubleshooting guide

[rrih]

Summary

• Adds a CSS Gradient Troubleshooting Guide to /tools/gradient-generator/
• Covers readability, muddy gradients, banding, and overpowering backgrounds with copyable CSS examples
• Regenerates public/sw.js through the normal build flow

Selected action

• Type: existing-page-expansion
• Automation mode: auto-pr
• Target: https://rrih.github.io/tools/gradient-generator/

Expected KPI impact

• Latest GSC snapshot shows /tools/gradient-generator/ at 401 impressions and 0 clicks, so this adds more search-intent-matching support copy to a high-impression page without changing ad density.
• The troubleshooting content targets practical CSS gradient implementation problems and keeps users in the CSS tools cluster.

Guardrails

• No AdSense account/settings changes
• No ad density changes
• No calculation, tax, labor, finance, DNS, secrets, or external production-scope changes
• Existing draft PRs Improve animation generator CTR copy #26 and Add overtime pay calculator #28 remain untouched

Verification

• GitHub preflight: success on attempt 1/4; open Improve animation generator CTR copy #26/Add overtime pay calculator #28 were draft, CLEAN, CI success
• bun test: 27 pass
• bun --bun tsc --noEmit: pass
• bun run lint: pass with existing 9 warnings
• bun run build: pass; 189 static pages generated
• bun run dev-check: pass; dev server reached Ready and was stopped
• git diff --check / git diff --cached --check: pass
• Local curl: /tools/gradient-generator/ contains the new troubleshooting heading, four troubleshooting entries, and CSS examples
• Browser plugin visual check: attempted, but in-app browser became unavailable after reconnect; curl/dev-server proof used as fallback

Refs #15