Make an image zoomable even when the feed didn't spell out where it lives

Tapping an image in a Ftrain article did nothing. The zoom affordance was
there, and the rendering path was the ordinary one -- the feed's own HTML, with
reader mode off -- but a tap went nowhere at all.

Tap detection works by wrapping every article image in a link to a scheme
nothing can navigate to, which the WebView then hands to
shouldOverrideUrlLoading. That href has to carry the complete image URL on its
own, so an image whose src was relative got no wrapper and no tap: better an
image that can't be zoomed, the reasoning went, than a link that goes nowhere.
Ftrain's <content:encoded> is entirely site-relative -- every image in it is
src="/art/uploads/..." -- so not one of them was zoomable.

That reasoning had an unexamined premise. The page itself resolves those srcs
perfectly well, against the base URL loadReaderHtml already hands
loadDataWithBaseURL: the article's own link. Resolving them the same way at
HTML-assembly time produces exactly the URL the WebView went and fetched, so
the wrapper can be built after all. Both reader surfaces now pass that base URL
down to the zoom pass. Where there genuinely is no base URL to resolve
against -- nothing else in the app loads reader HTML without one today -- the
old behavior stands and the image stays un-zoomable rather than acquiring a
dead link, and a data: or javascript: src is still refused however it was
reached, since the viewer could never open one.

This is why the report could only have come from a feed with reader mode off.
Reader mode never hits it: Readability rewrites every src to an absolute URL
before the extracted HTML reaches us, which is also why the same images zoom
fine the moment the feed's reader toggle is on.

TDD: site-relative and path-relative resolution, and the whole trip through
wrapReaderHtml, were written first against the unused parameter and went red on
assertion mismatches (3 of 38 in ImageZoomTest), green on the resolution
itself. The data:-src case was written at the same time and passed immediately
-- it pins existing behavior against the new code path rather than having
driven it.

Verified on the emulator against the reported article itself, subscribed live
from ftrain.com/feed.xml with reader mode off: on the previous build, a tap on
the header image left the screen byte-identical; on this one the same tap at
the same point opens the full-screen viewer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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