fix(web): serve SVG with image/svg+xml

[Alix-007]

📝 Description

Register .svg as image/svg+xml before serving embedded frontend assets so browsers receive the standards-compliant MIME type for SVG resources.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #1410

📚 Technical Context (Skip for Docs)

• Reference URL: [BUG] SVG files served with incorrect MIME type (image/svg instead of image/svg+xml) #1410
• Reasoning: Go can report .svg as image/svg on some platforms, so the embedded static server now forces the correct image/svg+xml mapping before requests are served.

🧪 Test Environment

• Hardware: PC
• OS: Windows 11
• Model/Provider: N/A
• Channels: N/A

📸 Evidence (Optional)

Click to view Logs/Screenshots

• go test ./web/backend -run 'Test(UnknownAPIPathStays404|MissingAssetStays404|RegisterEmbedRoutesRegistersSVGContentType)' -count=1

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[wj-xiao]

Thank you for your contribution.
But I'm sorry this PR duplicates 1429. I have to close this PR.