feat(homepage): surface Ghost Keys in the For Supporters card

[sanity]

Problem

Ghost Keys are unreachable from the homepage. The nav Donate button goes to /donate/, which links on to /ghostkey/ since #93, but nothing on the front page itself mentions them.

The constraint when we discussed this was that the homepage is already arguably too busy, so the fix shouldn't add weight.

Approach

It doesn't add any. The For Supporters card carried one link against two in each of its neighbours, with visible empty space below it:

card	links before	links after
For Users	2	2
For Developers	2	2
For Supporters	1	2

Filling that existing gap balances the row. Rendered page height is unchanged at 1944px on desktop.

The card's second sentence ("Your support helps build decentralized internet infrastructure that matters") was generic filler, so it makes way for a concrete one that also explains the new link. Without that, "Ghost Key" arrives as unexplained jargon on the front page.

Testing

hugo build clean. Verified in Chromium at 1280 and 390 wide: exactly one /ghostkey/ link on the page, desktop height unchanged at 1944px, and the three cards render evenly.

Note on framing

We'd discussed a "membership" reframe and recurring billing, which would change this copy. Neither is built, so this describes what actually exists today: a one-time donation that issues a key. Worth revisiting the wording if the recurring model ever lands.

[AI-assisted - Claude]

[sanity]

Code-first review lens run (read the code before the description, looking for mismatches). No substantive findings. It independently verified every falsifiable claim:

• Desktop height parity confirmed. Built both origin/main and this branch, measured in Chromium at 1280px: both exactly 1944px. The reviewer identified why, which is worth recording: .home-sections is a 3-column grid with default align-items: stretch (freenet.css:1031-1036), so row height is set by the tallest card. The two sibling cards were already two links deep, so the new link is absorbed into height they had already established.
• Mobile does grow, and this PR does not claim otherwise. At 390px the grid collapses to one column (freenet.css:1038-1042), so the extra link is not absorbed: 3558px → 3625px, +67px. Recording it here so the claim isn't read as broader than it is.
• check-links CI gate passes: scripts/check-links.py against the built site reports no broken internal links across 222 pages.
• goldmark rendering: both raw <a> tags render as separate un-mangled <p> elements at the pinned Hugo version.
• The removed sentence appears nowhere else in the repo. No meta description, test, or snapshot depended on it (the homepage layout emits no meta description at all).
• Exactly one /ghostkey/ link on the page.

Two non-blocking observations from the reviewer: stacked same-style links are the pre-existing pattern in the sibling cards rather than something new here, and link text is distinct so there's no ambiguous-link-name accessibility issue. The card's use of raw <a> rather than markdown link syntax is pre-existing inconsistency in the file, untouched by this PR.

[AI-assisted - Claude]