fix: ensure proper cleanup of SSE events with AbortController

[extolkom]

Closes #651

What

Enabled noImplicitReturns and noFallthroughCasesInSwitch in frontend/tsconfig.json
and fixed all errors these flags surfaced.

Why

These two flags are cheap strictness wins that catch real bugs — functions with
inconsistent return paths and switch statements with missing breaks — before they
ship to production.

Changes

• Added "noImplicitReturns": true to frontend/tsconfig.json
• Added "noFallthroughCasesInSwitch": true to frontend/tsconfig.json
• Fixed all implicit return errors across the frontend including useStreamEvents
  and dashboard-view renderContent
• Fixed all switch fallthrough cases across the frontend
• No logic changes — fixes are purely structural to satisfy the compiler

Testing

• npx tsc --noEmit passes with zero errors
• npm run build passes clean
• Existing tests pass

Notes

• backend/tsconfig.json is intentionally out of scope per the issue