[Frontend] Consolidate four different NEXT_PUBLIC_API_URL fallbacks (mix of /v1 and root)

[ogazboiz]

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Why this matters

Four places in the frontend define API_BASE_URL independently, with two different fallback shapes — some with /v1 and some without. This is the source of subtle bugs (e.g. issue #455 with the settings page calling ${baseWithV1}/health and getting /v1/health which doesn't exist).

Verified locations:

• frontend/src/app/streams/[id]/page.tsx:46 → ... || \"http://localhost:3001/v1\"
• frontend/src/app/settings/page.tsx:19 (named INDEXER_URL) → ... || \"http://localhost:3001/v1\"
• frontend/src/app/activity/page.tsx:13-15 → ... || \"http://localhost:3001\" (no /v1)
• frontend/src/components/TransactionTracker.tsx:59 → ... || \"http://localhost:3001/v1\"
• frontend/src/lib/api/_shared.ts:1 (canonical) → \"http://localhost:3001\" (no /v1) via getApiBaseUrl()

Some of these then concatenate /v1/streams/... directly, breaking depending on whether the env var ends in /v1.

Acceptance criteria

• Make every fetch use the shared helper in frontend/src/lib/api/_shared.ts (extend if needed) so there's exactly one place that knows the base URL shape
• All hardcoded fallback strings get removed from the four files above
• Behaviour identical whether NEXT_PUBLIC_API_URL is set with or without trailing /v1

Files to touch

• frontend/src/app/streams/[id]/page.tsx
• frontend/src/app/settings/page.tsx
• frontend/src/app/activity/page.tsx
• frontend/src/components/TransactionTracker.tsx
• frontend/src/lib/api/_shared.ts

Out of scope

• Reworking the SSE event subscribe URL