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

[ogazboiz]

clean — closes #651 (SSE/stream-detail fetches now wrap in AbortController for cleanup on unmount). all CI green. merging!

join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0

[ogazboiz]

approved! but a couple of conflicting merges landed first and now this branch is DIRTY. quick rebase and it can land:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

i'll auto-merge as soon as it's clean.

[extolkom]

ok thank you

[ogazboiz]

after rebase, the stricter tsconfig you enable in this PR (#651's noImplicitReturns / noFallthroughCasesInSwitch) now catches a real issue:

./src/components/TransactionTracker.tsx:152:13
Type error: Not all code paths return a value.

a switch or if-chain in TransactionTracker.tsx around line 152 has a branch that falls through without returning. add the missing return (or refactor to an exhaustive switch with a default). once that's fixed, the build should pass.