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.

3 files changed, 68 insertions, 5 deletions

diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
index 8694691..773cb91 100644
--- a/src/features/chat/index.ts
+++ b/src/features/chat/index.ts
@@ -4,7 +4,7 @@ export type {
   RenderGroup,
   ToolBatchEntry,
 } from "../../core/chunks";
-export { groupRenderedChunks, viewProviderRetry } from "../../core/chunks";
+export { groupRenderedChunks, resolveImageUrl, viewProviderRetry } from "../../core/chunks";
 export type { TurnMetricsEntry } from "../../core/metrics";
 export { isVisionModel } from "./model-select";
 export type { ChatTransport, HistorySync, HistoryWindow, MetricsSync } from "./ports";
diff --git a/src/features/chat/ui.test.ts b/src/features/chat/ui.test.ts
index b0aa6f0..5f8067d 100644
--- a/src/features/chat/ui.test.ts
+++ b/src/features/chat/ui.test.ts
@@ -628,6 +628,58 @@ describe("ChatView", () => {
     expect(img?.getAttribute("loading")).toBe("lazy");
   });
 
+  it("resolves a persisted image chunk's relative url against apiBaseUrl", () => {
+    // Persisted image chunks now carry a compact relative path (`/images/…`)
+    // served by the backend — prepend the API base to render them.
+    const chunks: RenderedChunk[] = [
+      {
+        seq: 1,
+        role: "user",
+        chunk: { type: "image", url: "/images/conv-123/abc-456.png", mimeType: "image/png" },
+        provisional: false,
+      },
+    ];
+
+    const { container } = render(ChatView, {
+      props: { chunks, apiBaseUrl: "http://localhost:24203" },
+    });
+
+    expect(container.querySelector("img")?.getAttribute("src")).toBe(
+      "http://localhost:24203/images/conv-123/abc-456.png",
+    );
+  });
+
+  it("passes a data URL through unchanged even with apiBaseUrl set (optimistic echo)", () => {
+    // The optimistic echo (what the FE just sent) is still a data URL; it must
+    // NOT be mangled by the base-URL prepend.
+    const dataUrl = "data:image/png;base64,iVBOR=";
+    const chunks: RenderedChunk[] = [
+      { seq: null, role: "user", chunk: { type: "image", url: dataUrl }, provisional: true },
+    ];
+
+    const { container } = render(ChatView, {
+      props: { chunks, apiBaseUrl: "http://localhost:24203" },
+    });
+
+    expect(container.querySelector("img")?.getAttribute("src")).toBe(dataUrl);
+  });
+
+  it("leaves a relative image url root-relative when apiBaseUrl is absent", () => {
+    // No apiBaseUrl → a browser resolves `/images/…` against the document origin.
+    const chunks: RenderedChunk[] = [
+      {
+        seq: 1,
+        role: "user",
+        chunk: { type: "image", url: "/images/conv-1/x.png" },
+        provisional: false,
+      },
+    ];
+
+    const { container } = render(ChatView, { props: { chunks } });
+
+    expect(container.querySelector("img")?.getAttribute("src")).toBe("/images/conv-1/x.png");
+  });
+
   it("renders a multi-chunk user message [text, image] and a transcription text", () => {
     // A non-vision model: the server persists the original image chunk AND a
     // transcription text chunk in the SAME user message — render both.
diff --git a/src/features/chat/ui/ChatView.svelte b/src/features/chat/ui/ChatView.svelte
index 8081951..cd69071 100644
--- a/src/features/chat/ui/ChatView.svelte
+++ b/src/features/chat/ui/ChatView.svelte
@@ -1,6 +1,6 @@
 <script lang="ts">
   import type { TurnProviderRetryEvent } from "@dispatch/wire";
-  import { groupRenderedChunks, type RenderedChunk, viewProviderRetry } from "../index";
+  import { groupRenderedChunks, resolveImageUrl, type RenderedChunk, viewProviderRetry } from "../index";
   import {
     interleaveTurnMetrics,
     viewCacheRate,
@@ -24,6 +24,7 @@
     onShowEarlier,
     thinkingKeyBase = 0,
     providerRetry = null,
+    apiBaseUrl = "",
   }: {
     chunks: readonly RenderedChunk[];
     turnMetrics?: readonly TurnMetricsEntry[];
@@ -44,6 +45,14 @@
      * newest attempt + delay, and is cleared when content resumes / turn ends.
      */
     providerRetry?: TurnProviderRetryEvent | null;
+    /**
+     * The HTTP API base URL (e.g. `http://localhost:24203`). Persisted image
+     * chunks carry a compact relative path (`/images/<conv>/<uuid>.png`); this
+     * base is prepended to render them. The optimistic echo's data URL and any
+     * absolute URL pass through unchanged (see `resolveImageUrl`). Defaults to
+     * "" (root-relative — a browser resolves `/images/…` against its origin).
+     */
+    apiBaseUrl?: string;
   } = $props();
 
   // True while a show-earlier page-in is awaited (disables the button).
@@ -96,15 +105,17 @@
 {#snippet chunkRow(rendered: RenderedChunk)}
   {#if rendered.role === "user"}
     <!-- User: a speech bubble, left-aligned. A user message may be multi-chunk
-         ([text, image, image, …]); each chunk renders in its own bubble (the
-         image's url is a base64 data URL or an https URL — render it directly). -->
+         ([text, image, image, …]); each chunk renders in its own bubble. A
+         persisted image chunk's url is a compact relative path (`/images/…`)
+         served by the backend — resolve it against the API base. The
+         optimistic echo's data URL (and any absolute URL) passes through. -->
     <div class="chat chat-start">
       <div class="chat-bubble chat-bubble-primary">
         {#if rendered.chunk.type === "text"}
           <p>{rendered.chunk.text}</p>
         {:else if rendered.chunk.type === "image"}
           <img
-            src={rendered.chunk.url}
+            src={resolveImageUrl(rendered.chunk.url, apiBaseUrl)}
             alt={rendered.chunk.mimeType ?? "pasted image"}
             loading="lazy"
             decoding="async"