50 MB attachment limit: insert oversized local files as machine + path references

[Michaelihc]

When a desktop attachment exceeds the 50 MiB (50 MB) upload limit, insert [machine][absolute path] into the composer instead and show “Large attachment inserted as path.”

This would let an agent read the file locally or transfer it via SSH/SCP when the source machine is reachable, for example through Tailscale.

Scope: desktop files whose absolute path is available. T3 Connect, browser/mobile clients, and unreachable source devices need a defined fallback; a path alone does not provide access.

Related upload-limit proposals: #15209 and its reader-compatibility prerequisite #15205. This proposes a path fallback rather than increasing the upload cap. No implementation yet.