feat(agent): support image attachments

[AnthonyRonning]

Summary

• add the standard multi-image composer experience to Agent Mode, including image-only sends, queueing, and persisted history previews
• send images directly to vision-capable models; give non-vision models opaque maple-attachment:// references that they can pass through a genuine read_image tool call
• store validated image bytes in an account- and session-scoped Maple cache, with bounded input sizes and cleanup on task deletion or history clearing

Closes #812.

Validation

• nix develop --no-update-lock-file .#ci -c ./scripts/ci/frontend.sh — 713 tests passed; typecheck/format/lint passed (13 existing lint warnings, 0 errors)
• nix develop --no-update-lock-file .#ci -c just rust-lint — passed
• nix develop --no-update-lock-file .#ci -c ./scripts/ci/rust.sh — 379 tests passed, 2 ignored
• nix develop --no-update-lock-file .#ci -c just desktop-build-debug-overlay — packaged macOS overlay built successfully
• local managed OpenSecret + billing + Maple Pro smoke:
  • GLM 5.2 called read_image itself on the opaque Maple attachment and returned an accurate description
  • Kimi K2.6 received the image directly and did not call read_image
  • image previews survived app relaunch; deleting the task removed its session cache

Packaged runtime smoke was performed on macOS. Windows and Linux packaged runtimes were not exercised locally.

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