Add an opt-in PDF download opener

[banjerluke]

Human comments

On my Android 13 eInk tablet (Boox Note Max), PDF links would open a side panel browser but wouldn't display, I assume due to platform limitations. The ideal flow on that device is to download the PDF, which sends it right into the Notes app where I can review and/or annotate it.

What was wrong

The file-opener system could select the built-in preview or a plugin viewer, but it had no first-class download action. This left users dependent on the browser’s PDF preview support, which is unreliable on some tablet browsers, without a persistent or one-off way to download the file.

What changed

• Added Download as a built-in opener choice for .pdf files under Settings → File openers.
• Preserved the existing built-in preview as the default when no PDF plugin is selected.
• Added one-off “Download PDF” and “Open preview” actions to thread file-link context menus, including long-press menus.
• Routed workspace, thread-storage, and host-file downloads through their existing content endpoints with an optional disposition=attachment query.
• Added safe Content-Disposition filenames with ASCII fallback and RFC 5987 UTF-8 encoding.
• Downloads now validate the HTTP response and show an error toast rather than saving an error response as a PDF.
• Explicit preview actions bypass the saved Download preference.
• No host-daemon protocol bump was needed because the server-to-daemon wire contract did not change.
• No new Plugin SDK surface or user-facing CLI setting was added. Existing agent file-reading commands remain unchanged.

How you verified

Added focused coverage for:

• PDF opener preference persistence and default behavior.
• Saved and one-off download selection.
• Explicit preview overrides.
• Workspace, thread-storage, and host-file download routing.
• Failed-download error handling.
• Unresolved workspace routing.
• Attachment query contracts and Content-Disposition headers.

Commands run:

• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/server --filter=@bb/server-contract
• pnpm exec turbo run test --filter=@bb/app -- src/lib/file-download.test.ts src/components/secondary-panel/useThreadFileTabs.test.ts src/components/settings/FileOpenersSettingsSection.test.tsx src/lib/plugin-slot-resolvers.test.ts
• pnpm exec turbo run test --filter=@bb/server -- test/hosts/daemon-file-response.test.ts test/public/public-thread-data.test.ts test/public/public-projects-local-host.test.ts
• pnpm exec turbo run test --filter=@bb/server-contract -- test/contract.test.ts
• pnpm exec turbo run build

The focused suites passed with 34 app tests, 90 server tests, and 35 server-contract tests. The full build completed successfully with 18/18 tasks.

Fixes #

AGENT GENERATED