Make the missing-GNU-rsync hint platform-specific and actionable

[jss367]

Problem

When a remote move is blocked because the machine has no GNU rsync, the error tells the user what is wrong but leaves them to work out how to fix it — including researching which package to install and how.

The message shown on Test connection:

⚠ SSH and the remote path are reachable, but no GNU rsync was found for the transfer. Install GNU rsync for your platform or set its path under Settings → Paths.

And the platform-support hint (vireo/platform_support.py:201):

Install GNU rsync and configure its executable under Settings → Paths.

Why it matters

This is a hard stop for a non-technical user. macOS ships Apple's openrsync, which reports itself as "rsync version 2.6.9 compatible" — so a user who checks rsync --version sees a plausible-looking rsync and concludes the app is wrong about it. Nothing in the message hints that the system binary is the problem, or that a second rsync is needed alongside it.

The app already knows everything required to give a precise instruction at the moment it renders this message:

• the platform is macOS (sys.platform)
• /usr/bin/rsync resolved but failed is_gnu_rsync — i.e. this is specifically the openrsync case, not the nothing-installed case; platform_support.py:199 already distinguishes "misconfigured" from "missing"
• Homebrew's install path /opt/homebrew/bin/rsync is already the third auto-detect candidate (vireo/move.py:270), so a brew install rsync will be picked up with no further configuration

Suggested fix

Make the hint platform- and state-specific:

• macOS + openrsync detected → "macOS ships Apple's openrsync, which can't transfer over SSH. Install GNU rsync with brew install rsync" — with a copy button on the command. Add a note that no further configuration is needed afterward.
• macOS + nothing found → same command.
• Linux → the distro package hint (apt install rsync / dnf install rsync).
• Windows → existing guidance.

Also worth reconsidering the "or set its path under Settings → Paths" suffix. It's the escape hatch, but it's listed as a co-equal option, which invites users to go hunting in Settings for a path they don't have yet. Leading with the install command and demoting the manual path to a secondary line would match what almost everyone needs.

Note

Once brew install rsync runs, auto-detect works with no user action — verified via the app's own resolver:

>>> move.resolve_rsync_bin()
'/opt/homebrew/bin/rsync'
>>> move.is_gnu_rsync('/opt/homebrew/bin/rsync')
True

So the entire remaining gap really is just the wording of the hint.