fix: reliable thumbnail resolution in hqify with async sddefault upgrade

[KrX3D]

Problem

hqify was unconditionally setting thumbnails to sddefault.jpg
(640×480), but YouTube only generates sddefault for videos uploaded
after a certain point. Older videos return a 404 for that filename,
which the TV browser renders as a grey placeholder instead of the
actual thumbnail.

Switching blindly to hqdefault (480×360) fixes the grey box but
sacrifices resolution for videos where sddefault does exist.

Solution

Two-phase approach:

1. Set hqdefault immediately — guaranteed to exist for every
   video ever uploaded, so no grey boxes under any circumstance.
2. Async HEAD-check sddefault — if the CDN responds 200,
   upgrade the same thumbnails[0] reference in-place to the
   higher resolution version.

Because thumbs holds a live reference into the item object and the
TV layout pipeline runs after multiple JS microtasks, the HEAD response
(against the same CDN already warm from page load) typically resolves
before the renderer reads the thumbnail, so most videos get the
640×480 version with no visible flash or fallback.

Also adds an early continue guard for tiles with no existing
thumbnail URL (some home page tiles are lazy-loaded and arrive with
an empty thumbnails array), preventing a crash in those cases.

Additionally, the original thumbnail URL from YouTube's JSON contains a
long query string (visible as ?sqp=...&rs=... on the URL). This is a
CDN signature tied to the exact filename it was issued for — copying it
onto a different filename causes a signature mismatch and the CDN returns
the grey placeholder regardless of whether the file actually exists.
Dropping the query string entirely avoids this; plain filename URLs like
hqdefault.jpg and sddefault.jpg work without any signing.

Before:

After:

[ice-fly]

Execllent fix, code comments, and contribution!