feat(quality): resolution/quality presets + Auto from pre-flight

Add a host-global quality knob (Discord-style) so the sharer can trade
resolution + bitrate for upload bandwidth. Quality is host-global by
design: one encode pipeline fans out to every viewer, so per-viewer
quality is out of scope (it would kill the broadcast fanout).

- New `--quality source|high|medium|low|auto` (ValueEnum) bundling a
  (max-height, bitrate, fps) tuple per preset; `auto` derives the preset
  from the saved bandwidth pre-flight (safe_mbps / viewer cap), falling
  back to `medium` when unmeasured. Default is auto; the interactive
  Host branch shows a picker when --quality is omitted (mirrors pick_app).
- `--max-height N` raw override; `--bitrate`/`--framerate` changed to
  Option so an explicit flag overrides just that field of the preset
  (precedence rule), leaving the rest of the preset intact.
- host/quality.rs: Preset table + resolve(); pure resolve_auto() split
  from the config read for testability. 5 unit tests lock preset
  pass-through, the Auto ladder, the unmeasured fallback, and override
  precedence.
- pipeline::build_args inserts `videoscale ! video/x-raw,height=N,
  pixel-aspect-ratio=1/1,width=[2,8192,2]` only for non-Source presets.
  PAR 1/1 forces a proportional downscale (without it videoscale keeps
  full width and squashes PAR — no bandwidth win); the even-stepped width
  range + even-rounded height satisfy H.264 4:2:0. EffectiveQuality is
  threaded capture -> wayland/x11 -> pipeline; max_viewers is now sized
  against the effective (post-preset) bitrate.
- Banner gains a quality line (preset label + ≤Np/kbps/fps + provenance).
- deps.rs checks `videoscale`; smoke-pipeline.sh adds a 1080->480
  downscale check asserting an even width below source.
- README: --quality preset table, Auto behavior, host-global note,
  --max-height/--bitrate/--framerate override precedence.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/common/deps.rs

@@ -16,6 +16,10 @@ pub fn check_host_binaries(display: DisplayServer, opts: &HostOpts) -> Result<()
     require("gst-launch-1.0")?;
     require("gst-inspect-1.0")?;
     require("pactl")?;
+    // videoscale (downscale for the quality presets) lives in plugins-base,
+    // the same package the gst tools need, so this rarely fails on its own —
+    // but check it for a clear error if a partial install is missing it.
+    require_gst_element("videoscale")?;
     require_gst_element("h264parse")?;
     require_gst_element("mpegtsmux")?;
     require_gst_element("pulsesrc")?;
@@ -136,6 +140,13 @@ fn install_hint_for_gst_element(name: &str) -> String {
             Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-plugins-good",
             _ => "the GStreamer X11 plugin (plugins-good)",
         },
+        "videoscale" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-base",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-plugins-base",
+            Some("fedora" | "nobara") => "gstreamer1-plugins-base",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-plugins-base",
+            _ => "the GStreamer plugins-base set",
+        },
         "h264parse" | "mpegtsmux" | "aacparse" => match distro.as_deref() {
             Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-bad",
             Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-plugins-bad",