fix(chat): send bounded ACP image attachments

[keh4l]

Summary

• route images from uploads, native/server pickers, drag-and-drop, and session attachments as inline ACP ImageContent
• keep ordinary resource uploads on the existing 2 MiB path while adding a separately bounded 20,000,000-byte image reader with regular-file and special-file checks
• treat mixed attachment batches as best-effort, clean image bytes from composer state after send/remove/unmount, and correct Grok's effective image capability based on verified inline-image support
• raise only /api/acp_prompt to a 32 MiB request-body limit so a 20 MB image can survive base64/JSON expansion without widening adjacent APIs

Addresses #358.

Verification

• pnpm eslint .
• pnpm test — 211 files, 2608 tests passed
• pnpm build — 32 static pages generated
• focused attachment tests — 29 passed
• focused Rust tests — image reader 6 passed, Grok capability 1 passed, ACP body limit 1 passed
• cargo check --no-default-features --bin codeg-server
• cargo test --no-default-features --bin codeg-server --lib — 1638 passed, 1 ignored
• cargo clippy --no-default-features --bin codeg-server --lib -- -D warnings
• cargo check --no-default-features --bin codeg-mcp
• cargo clippy --no-default-features --bin codeg-mcp -- -D warnings

Notes

• Desktop cargo check is not reported because this host lacks the GTK/GLib development packages required by glib-sys.
• Full-repository cargo fmt --check already fails on upstream main; this PR introduces no additional failing file compared with that baseline.
• No paid-model request was made. The Grok behavior was previously verified against 0.2.101/0.2.102 with inline ImageContent located outside the session worktree.