Preflight provider auth on send and unstrand hosted cold visitors

[badcuban]

Second batch of fresh-install fixes from the 0.3.2 UX audit.

What changed

• Send preflight (guide, don't block). When the selected provider instance is already known-unusable (not installed, or signed out), submitting holds the turn and immediately shows the same guidance the post-failure banner shows, plus a "Send anyway" escape hatch that re-enters the identical dispatch path. The draft, attachments, and timeline are untouched; the notice resets on thread switch. Previously this path burned 20-30s of visible reconnects and raw 401 Unauthorized before guidance appeared.
• Shared readiness cards. The sign-in card was extracted from ThreadErrorBanner into ProviderReadinessNotice.tsx (plus a not-installed sibling) so the pre-send and post-failure surfaces read identically. Providers with no known login command fall back to the install-and-connect card instead of naming a command that doesn't exist.
• Hosted cold visitors get a next step. The never-paired app.threadlines.dev screen now offers "Download the desktop app" (marketing download page) and "Pair this browser" (Settings → Devices), with copy naming where the setup link is actually minted (desktop: Settings → Devices → Add device — verified against the existing createServerPairingCredential dialog).
• Hosted add-project dead end. "Add a project" with no paired computer now explains the situation and routes to Devices instead of the bare "No environment is available" error. Desktop sessions with genuinely no environment keep the error wording.

Testing

• vp fmt, vp lint, vp run typecheck green.
• Unit: 137 pass across ChatView.logic + CommandPalette.logic (5 new preflight cases, 2 new guidance cases); full web unit suite 2045 pass.
• Browser: ChatView 108 pass (+1 integration test: signed-out send dispatches nothing and preserves the draft, then dispatches exactly once via Send anyway), HostedStaticStatusStates 1 pass (new: actions + hrefs), SettingsPanels 32 pass.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
threadlines	Skipped		Aug 7, 2026 1:22am