feat(river): warn mobile visitors that Freenet is desktop-only

[sanity]

Problem

A mobile visitor could land on /river, tap Install Freenet & Join River, and hit a dead end. Nothing on either page said Freenet is desktop-only, and the /quickstart installer made it worse: its os-install shortcode auto-selects an OS tab by device and maps iPhone/iPad → the macOS download and Android → the Linux install — so a phone user was shown a desktop download button that does nothing.

Approach

Be upfront on the landing page, and stop the installer from handing mobile users a broken download.

• /river hero — a platform line under the CTA, visible to everyone (so mobile users see it before tapping): "Runs on Windows, macOS, and Linux. No mobile app yet."
• /river "Where River is today" alpha box — an honest bullet: "Desktop only (Windows, macOS, Linux); no mobile app yet".
• /quickstart os-install shortcode — detect phones/tablets (including iPadOS 13+, which masquerades as desktop Mac, via maxTouchPoints) and replace the bogus desktop download with a clear notice: open the page on a computer, no mobile app yet. Includes an "On a computer? Show the install options" safety valve in case a desktop is misdetected. Desktop behavior is unchanged (detectOS / activateTab / tab handlers are byte-for-byte identical; the only removed line, activateTab(detectOS()), is preserved verbatim in the new else branch).

Phrased as "no mobile app yet" — honest about the current state without promising a timeline.

Testing

Verified end-to-end with Playwright:

• iPhone /quickstart: desktop tabs hidden (display:none), honest notice shown; the safety-valve button restores the tabs and re-runs detection.
• iPhone /river: platform line renders under the CTA with the correct text.
• Desktop /quickstart (control): tabs visible, notice hidden, correct OS tab auto-selected — unchanged from before.

Hugo build clean; no em-dashes.

Review

• Codex (external): clean — no functional regressions or discrete bugs.
• Adversarial read: clean — no false positives for Windows-touch laptops / Surface / MacBooks (the Mac+touch branch is gated on navigator.platform, and real Macs report maxTouchPoints === 0); isMobile() can't throw; safety valve verified. Three edge-case NITs (keyboard focus after the safety-valve click; a brief pre-hide tab flash; no-JS mobile falls back to the Linux default — pre-existing), all dismissed as not worth blocking.

Copy-only + progressive-enhancement JS; no high-risk surface.

[AI-assisted - Claude]