Bring the desktop app into the fork (Tauri + Electron, fully branded) #960

The fork was created from OpenCode without the desktop app. We want a fully-functional, fully-branded Altimate Code desktop app that exposes everything the fork has built, with no OpenCode branding leaks, and a publish-ready CI pipeline.

Scope

• Adopt the Tauri desktop shell (packages/desktop, primary) + Electron shell (packages/desktop-electron, secondary) and the SolidJS web UI (packages/app, packages/ui) from upstream v1.2.20.
• Bundle our opencode/altimate CLI as the Tauri sidecar so all server-side fork capabilities (SQL, dbt, FinOps, warehouse, altimate-core, the Altimate gateway) are exposed automatically.
• Rebrand every user-facing surface to Altimate Code (names, icons, colors, deep-link scheme, updater endpoints + key) — zero OpenCode leaks.
• Feature the Altimate LLM Gateway (altimate-backend) provider and remove the OpenCode Zen onboarding.
• Keep future upstream merges branded (un-skip packages, branding rules, altimate_change markers).
• Add a CI publish workflow (signed, auto-updating) — ready for org secrets.