fix(client): Linux auto decoder tries VAAPI before FFmpeg-Vulkan on desktop Mesa

Mesa now exposes Vulkan Video decode queues by default (and the session
binary opts RADV in for the Deck's sake), which silently moved every desktop
AMD/Intel box onto FFmpeg-Vulkan-on-Mesa under `auto` — user-reported
(CachyOS/KDE) to judder or error-streak into the software demotion while an
explicit VAAPI pick streams perfectly. Auto's hardware order is now
device-aware (`VulkanDecodeDevice::prefer_vulkan_over_vaapi`, fed
vendor id + device name by the presenter): Vulkan-first stays only where it
is the established right answer — NVIDIA (no usable VAAPI) and the Deck's
VanGogh (VAAPI dmabuf import chroma-fringes) — and everything else gets the
battle-tested zero-copy VAAPI first, with Vulkan as its fallback.

A mid-session Vulkan failure streak now also demotes to VAAPI before
software, so a broken Mesa Vulkan path can never strand a box with a
perfectly good VAAPI driver on CPU decode.

The GTK shell's decoder setting gains the missing "Vulkan Video" option
(values now mirror the console UI's auto/vulkan/vaapi/software) and drops
its pre-Vulkan "Automatic (VAAPI → software)" label.

Verified on the RTX 5070 Ti box (loopback session, auto → "Vulkan Video
hardware decode active", 60 fps); policy locked by unit test; clippy -D
warnings + pf-client-core/pf-presenter tests green on Linux.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/linux/src/ui_settings.rs

@@ -29,7 +29,7 @@ const COMPOSITORS: &[&str] = &["auto", "kwin", "wlroots", "mutter", "gamescope"]
 /// Codec setting values (persisted) paired with their display labels below.
 const CODECS: &[&str] = &["auto", "hevc", "h264", "av1"];
 const CODEC_LABELS: &[&str] = &["Automatic", "HEVC (H.265)", "H.264 (AVC)", "AV1"];
-const DECODERS: &[&str] = &["auto", "vaapi", "software"];
+const DECODERS: &[&str] = &["auto", "vulkan", "vaapi", "software"];
 /// Touch-input model values (persisted) paired with their display labels below — the
 /// cross-client set (Android/Apple). Only meaningful on a touchscreen (Deck/tablet).
 const TOUCH_MODES: &[&str] = &["trackpad", "pointer", "touch"];
@@ -324,10 +324,12 @@ pub fn show(
         &dialog,
         inline,
         "Video decoder",
-        "Automatic tries VAAPI hardware decode, then software",
+        "Automatic picks the best hardware decode for this GPU (VAAPI on AMD/Intel, \
+         Vulkan Video on NVIDIA), falling back to software",
         &[
-            "Automatic (VAAPI → software)",
-            "Hardware (VAAPI)",
+            "Automatic (hardware → software)",
+            "Vulkan Video",
+            "VAAPI",
             "Software",
         ],
     );