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.

0 files changed, 0 insertions, 0 deletions