feat: record at the source's size, only once capture works

Encoders and the segment output are created when the capture source first
has a picture, at its own size (portrait monitors record portrait) unless a
resolution is configured, so nothing is written while waiting for a source.
SPEC.md records both decisions; validation docs cover how sessions should end
when the game closes.

crates/clipbin-core/src/settings.rs

@@ -21,8 +21,9 @@ impl fmt::Display for VideoCodec {
 /// In-memory defaults from SPEC.md. Persistence and editing come with the recorder.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Settings {
-    pub width: u32,
-    pub height: u32,
+    /// A fixed recording size. `None` records at the captured window's or
+    /// monitor's own size, following its orientation.
+    pub resolution: Option<(u32, u32)>,
     pub frames_per_second: u32,
     pub codec: VideoCodec,
     pub system_audio_enabled: bool,
@@ -38,8 +39,7 @@ pub struct Settings {
 impl Default for Settings {
     fn default() -> Self {
         Self {
-            width: 1920,
-            height: 1080,
+            resolution: None,
             frames_per_second: 60,
             codec: VideoCodec::H264,
             system_audio_enabled: true,