refactor(dispatch): install only published releases; flatten target setup

[bobleer]

Why

Two things, both about the dispatch target setup dialog.

1. Remove the ability to compile BitFun on the target. It required a Rust toolchain, a C compiler, git, ~6 GB of free space, and tens of minutes on a machine the user only meant to run one task on. It existed solely to paper over targets that no published binary fits; those targets are now reported as unusable instead.

2. The form was too deeply nested. Showing one project path meant passing through four levels of boxes: modal → section card with a filled header bar → inner card → code pill.

What changed

Only the published binary install path remains

Removed end to end, so the capability cannot come back through an unused door:

• The dispatch_install_cli_source_start command, its Tauri adapter, the Server Host route, and the CLI/Desktop peer table entries
• The Desktop controller-source archiver (archive_controller_source / controller_source_root)
• install_cli_source_start, install_cli_source_archive_start, both remote build script generators, and source_build_availability
• The probe's cargo / git / cc / free-space detection, the DispatchSourceBuild type, and the strings in all three locales

The probe now fills install_error in the two branches that previously returned nothing and leaned on the source-build card, so a target with no compatible release explains itself rather than failing later at submit. The UI gains an installUnavailable message and keeps "Use this target" disabled. A new contract test pins that no routing table or client API mentions dispatch_install_cli_source_start.

Flatten the setup form

The dialog loses the install poll/cancel and console machinery along with the source build — nothing in this dialog starts an install anymore; submit does.

What remains is one flat stack of sections separated by hairlines, with a border reserved for the two things that are actually interactive (the revision input and the approval options). The automatic-setup and model-sync panels fold into the target check they belong to, so five sections become three and the whole form fits without scrolling.

Verification

• cargo check: bitfun-services-integrations / bitfun-core / bitfun-server / bitfun-cli / bitfun-desktop all pass; no new clippy warnings
• Rust tests: dispatch_ssh 31, core dispatch 37, server route 3, CLI deny 1 — all green
• web-ui: 2565 tests green, tsc and eslint clean
• Rendered the old and new styles side by side to compare the actual layout

Note

installCliStart / installCliPoll / installCliCancel are kept in dispatchApi: their backend commands still exist and belong to the binary install path, they just have no frontend caller right now (installCliStart already had none before this change).