Desktop launcher: per-vendor model selection from the server catalog #900

Problem

When launching a hosted agent in the web UI, most harnesses offer a full selectable list of models. The desktop launcher only offers a curated model list for Claude and Codex; every other vendor shows just "Default" + free-text. Bring the desktop launcher to parity with the web UI's model list.

Findings

• Desktop launcher already has a model picker (the Agent chip flyout, src/Capacitor.App/Views/LauncherPaneView.axaml.cs RebuildRows), fed by a hardcoded list in src/Capacitor.App/Services/HostedHarnessCatalog.cs (only claude + codex populated).
• The web UI feeds its dropdown from the server-side models.dev catalog. That catalog is already exposed to remote clients over REST: GET /api/agents/model-options (auth-gated), returning { vendor: [{ value, label }] }.
• The desktop app already has an authenticated HTTP read path: src/Capacitor.App/Services/AuthenticatedServerReads.cs (used by RemoteAgentsService). No kcap-server change is required.
• Note: the web UI launcher has no context-window-size selector — context window is a display-only occupancy metric, not a launch input. So this is model selection only (effort already exists as its own chip).

Proposed change

Add an authenticated model-catalog client that GETs /api/agents/model-options, cache it in memory, and feed it into the launcher model flyout for all vendors (keeping Default / Custom… rows). Fall back to the existing curated list when offline / not yet loaded.

Acceptance

• Opening the launcher model picker for any vendor with server-side models shows the same list as the web UI.
• Offline or signed-out falls back gracefully to the curated/default list.