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.

2 files changed, 11 insertions, 0 deletions

diff --git a/src/app/App.svelte b/src/app/App.svelte
index f41d7ba..7d70f67 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -516,6 +516,7 @@
 							onShowEarlier={handleShowEarlier}
 							thinkingKeyBase={store.activeChat.thinkingKeyBase}
 							providerRetry={store.activeChat.providerRetry}
+							apiBaseUrl={store.httpBase}
 						/>
 					{/key}
 				</div>
@@ -610,6 +611,7 @@
 			closeChat={closeRunChat}
 			stopRun={stopHeartbeatRun}
 			onClose={() => (heartbeatRun = null)}
+			apiBaseUrl={store.httpBase}
 		/>
 	{/key}
 {/if}
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 554d5e0..8353820 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -157,6 +157,12 @@ export interface AppStore {
   readonly activeConversationId: string | null;
   /** The workspace currently in view (URL slug); tabs are filtered to it. */
   readonly activeWorkspaceId: string;
+  /**
+   * The resolved HTTP API base URL (e.g. `http://localhost:24203`). Used to
+   * resolve relative image URLs served by the backend (`/images/…`) into
+   * absolute URLs for `<img src>`.
+   */
+  readonly httpBase: string;
   readonly activeChat: ChatStore;
   readonly models: readonly string[];
   /** Per-model metadata (contextWindow, etc.) from `GET /models`. */
@@ -1127,6 +1133,9 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
     get activeWorkspaceId(): string {
       return activeWorkspaceId;
     },
+    get httpBase(): string {
+      return httpBase;
+    },
     setActiveWorkspace(workspaceId: string): void {
       activeWorkspaceId = workspaceId;
       // Reset to a fresh draft scoped to the new workspace so a new chat is