perf: reduce input tarpitting and speed up large-display rendering

Input tarpitting fixes:
- terminal.ts: batch stdin writes with 10ms coalescing window (flushes
  immediately for large payloads like paste), replacing per-keystroke
  WebSocket messages with fewer, larger frames
- server.go: replace per-message time.After() with a reusable timer to
  eliminate GC pressure from repeated key input
- server.go: coalesce queued stdin writes (up to 4KB) into a single PTY
  write to reduce syscall overhead

Screenshot/rendering pipeline optimizations:
- tracker.go: stop forcing empty cells to space; let exporters decide
  what to render, drastically reducing work for mostly-blank terminals
- tracker.go: use clear() for dirty map instead of delete-in-loop
- svg_exporter.go: skip visually empty cells (blank glyph, default BG,
  no reverse/underline); still render background rects for colored or
  reverse-video cells
- png_exporter.go: add color parsing cache to avoid redundant hex
  parsing per cell; add empty cell fast-path; short-circuit blend math
  for coverage 0 and 255

Dashboard thumbnail concurrency:
- server.go: replace single-flight screenshot fetching with limited
  parallelism (2-4 concurrent requests based on hardwareConcurrency)
  so large dashboards with many tiles update faster

Also fixes typo in dashboard JS (tetagBySlug -> etagBySlug) that
silently broke ETag caching for screenshot refreshes.

Bumps version to 1.3.32.

internal/terminalstate/tracker.go

@@ -74,9 +74,7 @@ func (t *Tracker) Feed(data []byte) error {
 	if len(t.screen.Dirty) > 0 {
 		t.changeCounter++
 		// Clear dirty set so subsequent feeds detect new changes
-		for k := range t.screen.Dirty {
-			delete(t.screen.Dirty, k)
-		}
+		clear(t.screen.Dirty)
 	}
 	return nil
 }
@@ -108,9 +106,6 @@ func (t *Tracker) Snapshot() Snapshot {
 		for col := 0; col < t.screen.Columns; col++ {
 			raw := t.screen.Buffer[row][col]
 			data := raw.Data
-			if data == "" {
-				data = " "
-			}
 			line[col] = Cell{
 				Data:       data,
 				FG:         colorToString(raw.Attr.Fg),