Replace Translate-based centering with animation.Origin; add zoom position config

The previous "fix" for Scale's crop bias paired Scale with a computed
Translate, on the mistaken assumption that Scale's default pivot is the
top-left corner. It isn't: pixlet's docs say the default origin is
already the center (0.5, 0.5). That Translate was sized assuming Scale
had no self-centering at all, so it overcorrected, visibly skewing the
zoom toward the bottom-right -- the opposite of the bug it meant to fix.

animation.Origin does directly and correctly what that Translate was
clumsily approximating, so the Translate (and its rounding="none"
workaround) is removed entirely in favor of Origin. This also means
"strong" no longer needs the 1.45 safety cap -- restored to 1.7, since
the black-border risk was specific to the Translate formula, not Scale +
Origin (verified via the calibration tool at several off-center origins).

Added a "Zoom position" schema option: Center (default) or Random. Random
picks from a handful of preset off-center spots, chosen per-track from
subsonic_id the same way the old corner-selection logic worked, so a
still-playing track doesn't jump between spots on every refresh.

Verified: no black border, no popping (confirmed via frame-delta
analysis -- smooth throughout, no Translate means rounding is moot), and
payload well under the 460KB limit at every zoom level/position
combination, against the live station.

README.md

@@ -23,6 +23,9 @@ app's config when adding it to a device.
   muted color for the time/temp line.
 - **Album art zoom** — how far the album art's breathing zoom effect goes
   in: Off, Subtle, Moderate (default), or Strong.
+- **Zoom position** — where the zoom centers on: Center (default), or
+  Random (a handful of preset off-center spots, picked per-track so a
+  still-playing track doesn't jump between spots on every refresh).
 
 ## How it works
 
@@ -46,30 +49,39 @@ app's config when adding it to a device.
   attempts produced a real, on-device black border; see the fuller
   history below.
 
-  `animation.Scale`'s pivot defaults to the *center* of the widget per
-  pixlet's own docs, so Scale alone is already close to centered — just
-  slightly skewed toward the top-left as the zoom amount increases.
-  Paired with a computed `animation.Translate` to cancel out that residual
-  skew. Combining Scale with Translate at all was tried once before and
-  reverted after it produced a periodic rendering "pop" roughly every
-  0.7-1s — confirmed with both WebP and GIF output, independent of zoom
-  amplitude. Root-caused to pixlet's own source
+  Where the zoom centers on is controlled via `animation.Origin`, which
+  works correctly and does real work here — contrary to an earlier,
+  mistaken belief in this app's own history that it "had no effect" (that
+  came from testing it set to its own default value and comparing the
+  result against itself, which is of course identical either way). The
+  "Random" zoom position picks from a handful of preset off-center spots
+  (`ZOOM_RANDOM_SPOTS`), chosen per-track from `subsonic_id` the same way
+  the old corner-selection logic worked, so a still-playing track doesn't
+  jump between spots on every `NOWPLAYING_TTL` refresh. Those spots are
+  kept modest (0.3-0.7 of the image's own bounds, not the full 0.0-1.0
+  range) and verified safe (no black border, even at the strongest 1.7x
+  zoom) via the calibration tool; more extreme corners weren't tested and
+  may not be safe.
+
+  An earlier version tried to fix a *different* problem — `animation.Scale`
+  zooming in asymmetrically toward one corner — by pairing Scale with a
+  *computed* `animation.Translate`, reasoning (wrongly) that Scale's
+  default pivot is the top-left corner. It isn't: pixlet's own docs say
+  the default origin is already the center. That Translate, sized
+  assuming Scale had no self-centering at all, overcorrected and visibly
+  skewed the zoom toward the *opposite* corner instead. It also introduced
+  a periodic rendering "pop" (confirmed with both WebP and GIF output,
+  independent of zoom amplitude) root-caused to pixlet's own source
   (`render/animation/translate.go`): a Translate's x/y are rounded to the
-  nearest whole pixel every frame by default, while Scale is never
-  rounded, so a continuously-changing Scale combined with a rounded
-  Translate holds still for several frames and then jumps a whole pixel
-  at once. Passing `rounding = "none"` on the `Transformation` disables
-  that snapping and fixes it — confirmed via frame-by-frame pixel-delta
-  analysis (the amount of change is now even from frame to frame, instead
-  of long still stretches punctuated by sudden jumps). "strong" is capped
-  at 1.45 (not pushed further): above ~1.46 the centered crop starts
-  revealing a real black border in the bottom-right corner, verified with
-  the calibration tool — this is the safe ceiling, not an arbitrary
-  choice. Verified payload stays well under the 460KB device limit at
-  every zoom level, measured against the live station (though with
-  `rounding = "none"`, WebP can no longer dedupe as many near-identical
-  frames, so payloads run larger than the "pop" version did — ~370KB
-  vs. ~220-320KB — still comfortably within budget).
+  nearest whole pixel every frame by default, while Scale never rounds,
+  so a continuously-changing Scale combined with a rounded Translate
+  holds still for several frames and then jumps a whole pixel at once.
+  Since `animation.Origin` already does directly and correctly what that
+  Translate was clumsily approximating, the Translate was removed
+  entirely rather than patched further (e.g. with `rounding = "none"`,
+  which does fix the popping but wasn't needed once Origin replaced it).
+  Verified payload stays well under the 460KB device limit at every zoom
+  level and position, measured against the live station.
 
   **Why a loop, not a one-shot reveal, and why it's short:** a one-way
   version looked jumpy in practice — since the app re-renders on