feat(chat): compress images at attach time (JPEG re-encode + rescale) with a per-image toggle

Staged images now default to a compressed variant: LOSSLESS sources
(PNG/BMP) are re-encoded to JPEG at q0.85, rescaled to cap the longest
edge at 2048px (never upscaled, high-quality smoothing, alpha flattened
onto white), and used only under a smaller-wins rule. JPEG/WebP/GIF are
sent verbatim. Each staged thumbnail gets a Compress checkmark (default
ON, reversible until send, cached so flipping never re-encodes); OFF
sends the original untouched bytes. Conversion runs async at staging and
any failure silently falls back to the original — sending is never
blocked or broken.

Pure decision logic lives in features/chat/logic/image-compress.ts
(format selection, dimension math, smaller-wins); the canvas encode is a
thin injected edge (features/chat/adapter/image-encode.ts, Composer prop
encodeJpeg). No wire-contract, send-path, or backend changes; complements
the backend's body-budget eviction safety net by shrinking payloads at
the source.

Verification: typecheck 0 errors, vitest 1720/1720 (twice), biome clean,
build succeeds. New tests: 12 pure-logic, 2 encoder-edge, 6 component.

1 files changed, 20 insertions, 0 deletions

diff --git a/src/features/chat/adapter/image-encode.test.ts b/src/features/chat/adapter/image-encode.test.ts
new file mode 100644
index 0000000..dbaa8ae
--- /dev/null
+++ b/src/features/chat/adapter/image-encode.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { convertToJpeg } from "./image-encode";
+
+describe("convertToJpeg (canvas edge — real environment, no mocks)", () => {
+  it("skips non-convertible formats without touching the canvas", async () => {
+    const jpeg = new File([new Uint8Array([0xff, 0xd8])], "photo.jpg", { type: "image/jpeg" });
+    await expect(convertToJpeg(jpeg)).resolves.toBe(null);
+    const gif = new File([new Uint8Array([0x47])], "anim.gif", { type: "image/gif" });
+    await expect(convertToJpeg(gif)).resolves.toBe(null);
+    const unnamed = new File([new Uint8Array([0x00])], "blob", { type: "" });
+    await expect(convertToJpeg(unnamed)).resolves.toBe(null);
+  });
+
+  it("falls back to null on decode failure (no real canvas in this environment)", async () => {
+    const png = new File([new Uint8Array([0x89, 0x50])], "shot.png", { type: "image/png" });
+    // jsdom has no `createImageBitmap` → the decode throws → the failure path
+    // must resolve to `null` (never reject), so sending is never blocked.
+    await expect(convertToJpeg(png)).resolves.toBe(null);
+  });
+});