proxy,ui-svelte: add request/response capturing (#508)

Add saving request and response headers and bodies that go through
llama-swap in memory.

- captureBuffer added to configuration. Captures are enabled by default.
- 5MB of memory is allocated for req/response captures in a ring buffer.
Setting captureBuffer to 0 will disable captures.
- UI elements to view captured data added to Activity page. Includes
some
QOL features like json formatting and recombining SSE chat streams
- capture saving is done at the byte level and has minimal impact on
llama-swap performance

Fixes #464 
Ref #503

proxy/proxymanager_api.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"sort"
+	"strconv"
 	"strings"
 
 	"github.com/gin-gonic/gin"
@@ -31,6 +32,7 @@ func addApiHandlers(pm *ProxyManager) {
 		apiGroup.GET("/events", pm.apiSendEvents)
 		apiGroup.GET("/metrics", pm.apiGetMetrics)
 		apiGroup.GET("/version", pm.apiGetVersion)
+		apiGroup.GET("/captures/:id", pm.apiGetCapture)
 	}
 }
 
@@ -250,3 +252,20 @@ func (pm *ProxyManager) apiGetVersion(c *gin.Context) {
 		"build_date": pm.buildDate,
 	})
 }
+
+func (pm *ProxyManager) apiGetCapture(c *gin.Context) {
+	idStr := c.Param("id")
+	id, err := strconv.Atoi(idStr)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid capture ID"})
+		return
+	}
+
+	capture := pm.metricsMonitor.getCaptureByID(id)
+	if capture == nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "capture not found"})
+		return
+	}
+
+	c.JSON(http.StatusOK, capture)
+}