fix(opencode): name the release when upgrading OpenCode

Since opencode 1.18.x, `POST /global/upgrade` requires a `target` semver in
the body. OpenChamber sent an empty object, so every "Update OpenCode" click
came back 400. The rejection arrives as `{name, data:{message}}`, which has
no `error` field, so the user was left with the bare status text: "Bad
Request".

Resolve the target from the latest release — the same lookup the upgrade
prompt already uses to decide there is anything to offer — and fail with an
explicit code when it cannot be resolved, rather than sending a body opencode
is guaranteed to reject. Read the upstream rejection message so a refused
upgrade explains itself.

The VS Code extension carries its own copy of this flow and had the same two
defects; both are fixed there.

fixes #3121

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to this project will be documented in this file.
 - Git/Worktrees: session menus can now move an idle session and its sub-sessions into an existing worktree, and opening the target list discovers worktrees created outside OpenChamber without a restart (thanks to @mattv8).
 - Files: opening a file over 5,000 lines is no longer blocked — the line-count guard now allows up to 20,000 lines, letting large files reach the virtualized full-file preview instead of being rejected at the open step (thanks @gaojunran).
 - Usage: GitHub Copilot now shows a single AI Credits window, matching Copilot's token-based quota, in place of the old Chat Requests and Completions windows (thanks to @jakoss).
+- Updates: "Update OpenCode" no longer fails with a bare "Bad Request". OpenChamber now names the release to install, which recent OpenCode versions require, and when an update is refused the reason from OpenCode is shown instead of the HTTP status. This affects setups where OpenChamber runs an OpenCode you installed yourself; the desktop app bundles OpenCode and never offered the button.
 - Settings: fixed the Cloudflare Tunnel download link shown when cloudflared is not installed (thanks to @AyoubAchour).
 - Git: picking a remote branch such as `origin/main` in the branch selector now switches you to that branch instead of leaving the repository on a detached `HEAD` with no branch name.
 - Desktop: "Restart to Update" no longer looks dead when the update cannot be installed — the update window now shows the reason, including when the running copy was not installed from an official signed release, and the button stays available to retry.