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.

1 files changed, 46 insertions, 0 deletions

diff --git a/src/core/chunks/image-url.test.ts b/src/core/chunks/image-url.test.ts
new file mode 100644
index 0000000..8a79c09
--- /dev/null
+++ b/src/core/chunks/image-url.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { resolveImageUrl } from "./image-url";
+
+const BASE = "http://localhost:24203";
+
+describe("resolveImageUrl", () => {
+  it("returns a data URL as-is (the optimistic echo / a pasted image)", () => {
+    const dataUrl = "data:image/png;base64,iVBORw0KGgo=";
+    expect(resolveImageUrl(dataUrl, BASE)).toBe(dataUrl);
+  });
+
+  it("returns an absolute http URL as-is", () => {
+    const abs = "https://example.com/img.png";
+    expect(resolveImageUrl(abs, BASE)).toBe(abs);
+  });
+
+  it("prepends the api base to a relative /images/ path", () => {
+    expect(resolveImageUrl("/images/conv-123/abc-456.png", BASE)).toBe(
+      "http://localhost:24203/images/conv-123/abc-456.png",
+    );
+  });
+
+  it("does not double the slash when the base has a trailing slash", () => {
+    expect(resolveImageUrl("/images/c/x.png", "http://localhost:24203/")).toBe(
+      "http://localhost:24203/images/c/x.png",
+    );
+  });
+
+  it("adds a leading slash to a path-relative url without one", () => {
+    expect(resolveImageUrl("images/c/x.png", BASE)).toBe("http://localhost:24203/images/c/x.png");
+  });
+
+  it("returns the relative path as-is when apiBase is empty (root-relative)", () => {
+    // A browser resolves a root-relative `/images/…` against the document origin.
+    expect(resolveImageUrl("/images/c/x.png", "")).toBe("/images/c/x.png");
+  });
+
+  it("handles a relative path with an empty apiBase (path-relative without slash)", () => {
+    expect(resolveImageUrl("images/c/x.png", "")).toBe("/images/c/x.png");
+  });
+
+  it("returns a data URL as-is even with an empty apiBase", () => {
+    const dataUrl = "data:image/jpeg;base64,AAAA";
+    expect(resolveImageUrl(dataUrl, "")).toBe(dataUrl);
+  });
+});