Fix guide images, and the service class the timetable hides in an icon

[ialakey]

Two things reported as "it doesn't work", both of which turned out to be a
column of someone else's markup being read the wrong way.

Guide images were placeholders

srb.guide runs Hugo behind lazysizes. Every image looks like this:

<img class="lazyload blur-up"
     src=/media/soko_hu_6409ba5140e498ce.webp      <!-- blurred stub, 462 bytes -->
     data-srcset="…480w, …1080w, …1280w, …2268w"   <!-- the real files -->
     data-src=…>

The scraper read src, so 257 of the guide's 283 images were blurred stubs
of a few hundred bytes — the worst 20 pixels wide. It now prefers data-srcset
and takes the widest variant a phone can use, capped at 1280px: the 2268px
original is twelve times the bytes of the 480w for pixels no phone here has.
assets/data/guide.json is re-scraped with the fixed tool; a sample of ten
images went from ~460 bytes to 10–95 KB.

On screen:

• Tap an image to open it full screen. Pinch and double-tap were always
  there but invisible, so the viewer has zoom buttons too — four steps of 1.6x,
  the last step out returning to the fitted size, and a double-tap back to
  fitted resets the buttons with it.
• Inline images are capped at half a screen; a portrait screenshot at full
  width used to push the rest of the article off the page.
• The inline image and the viewer both report a failed load instead of leaving
  a gap, and the viewer shows download progress.

Note for after the merge: the app prefers the guide it downloads from GitHub
over the bundled copy, so existing installs correct themselves within a day of
this landing on master (or immediately via Settings → check now).

The class of train never showed

The route card and the station board each have a badge for the service class,
and it was always absent. The timetable renders that column as
<img title="REGIO VOZ">, not text, so the cell's text was empty and the badge
was never built. Cells now fall back to the titles of the images they contain,
which also recovers the Ponuda column the parser had been dropping — travel
classes, bicycle carriage, and whether a reservation is required. That reads
under the badges on a route card:

03:23 ──── 06:36 ──── 09:59
[Train 2901] [REGIO VOZ]
Prvi razred · Drugi razred · Bicikla - Ograničen broj mesta

Verification

• flutter analyze clean; 89 tests pass, including 8 new ones for the srcset
  picker
• The live timetable test now asserts both recovered columns arrive, since
  neither has a visible failure mode — they just go quiet if the markup changes
• Checked on an Android 16 emulator: guide photos render sharp at full size,
  tap opens the viewer, the zoom buttons step in and back to fitted, and
  Beograd Centar → Niš shows four runs with their class and offer