feat(vision): resolve persisted image URLs against the API base

Images are now stored on disk under tmp (not SQLite) and served via
GET /images/:conversationId/:imageId. Persisted ImageChunk.url is a compact
relative HTTP path (/images/<conv>/<uuid>.png) instead of a base64 data URL.
No wire/transport-contract type change (behavior only) — re-mirrored the
delta notes.

- New pure resolveImageUrl(url, apiBase) helper (core/chunks/image-url.ts,
  +8 tests): data/absolute URLs pass through; relative paths are prepended
  with the API base (no double slash; empty base -> root-relative). Exported
  from core/chunks + re-exported from features/chat.
- ChatView: new apiBaseUrl prop; <img src> uses resolveImageUrl. The
  optimistic echo's data URL passes through; persisted relative paths
  resolve against the base. +3 tests.
- AppStore exposes httpBase (getter); App.svelte passes apiBaseUrl into
  ChatView and the heartbeat RunModal (also renders image chunks).

Verification: svelte-check 0/0; vitest 959/959 (run twice, +11); biome
clean; vite build OK. See backend-handoff.md §2j-update-2.

Not merged or pushed.

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