feat(session): the stats overlay scales with the display's DPI

The stream chrome — stats OSD, capture hint, start banner, resize label —
was hardcoded at 14 px with 12/10/8 px insets. The overlay composites into
the swapchain 1:1 in PHYSICAL pixels, so on a 4K panel at 200 % all of it
rendered at half its intended physical size: fine on a 1080p monitor, a
squint on a HiDPI laptop.

FrameCtx now carries a scale — SDL's window display scale (DPI × the
display's content scale) times a PUNKTFUNK_OSD_SCALE preference — and every
metric moved into a `base` module that is multiplied by it. Re-read per
frame and quantized into the damage key, so dragging the window to a
differently-scaled monitor re-renders at the new size rather than keeping
the stale one. Sanitized because SDL returns 0.0 when it cannot resolve the
window's display, which would collapse the panel to nothing.

Two details worth keeping: the face is re-derived at the scaled size rather
than the canvas transformed, because Skia rasterizes glyphs at the
requested size where a magnified 14 px bitmap would be mush; and the long
capture hint is fit-clamped to the window — at 2× it is wider than a 1080p
screen, so scaling it naively would have traded a small OSD for a truncated
one. Linux and Windows share this path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 744467d13a28b91ba88a23d6038da70263e9b502)

docs-site/content/docs/configuration.md

@@ -167,6 +167,7 @@ A few knobs are read by the native **clients**, not the host:
 | Setting | Values | Meaning |
 |---|---|---|
 | `PUNKTFUNK_DECODER` | `software` · `vaapi` · `vulkan` (Linux) · `d3d11va` (Windows) | Force the decode path. Default auto-selects hardware per vendor (Linux: VAAPI on Intel/AMD, Vulkan Video on NVIDIA and the Steam Deck; Windows: Vulkan Video on NVIDIA/AMD, D3D11VA on Intel and others) with a software fallback. |
+| `PUNKTFUNK_OSD_SCALE` | multiplier, e.g. `1.5` *(default `1`)* | Size of the in-stream overlay — the stats OSD, the capture hint and the start banner. They already follow your display's scaling setting (200 % display → twice the pixels), so set this only to nudge that: bigger for a TV across the room, smaller if your compositor reports an aggressive scale. Clamped to 0.5×–4×, and a line that would run off the screen is shrunk to fit. |
 
 ## Bitrate