feat(headroom): add proxy lifecycle management + dashboard UI

Build on the optional Headroom Token Saver from Carmelo Campos
(PR: feat: add optional Headroom token saver). Add managed start/stop
of the local headroom proxy from the dashboard, install detection,
status probing, and a simplified Token Saver UI.

- detect headroom CLI + python>=3.10, probe proxy /health
- spawn/stop proxy as a detached, pid-tracked process
- /api/headroom/{status,start,stop} routes, gated local-only in dashboardGuard
- one-click Start/Stop Headroom modal, no manual config needed
- claude<->openai shape conversion for /v1/compress via 9router translators

Thanks to Carmelo Campos (@carmelogunsroses) for the original Headroom integration.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/rtk/systemInject.js

@@ -0,0 +1,98 @@
+// Shared system-prompt injector: appends an instruction into the system message of
+// the final request body, dispatching by format so it works for translated and
+// native-passthrough flows. Used by caveman.js and ponytail.js.
+
+import { FORMATS } from "../translator/formats.js";
+
+const SEP = "\n\n";
+
+export function injectSystemPrompt(body, format, prompt) {
+  if (!body || !prompt) return;
+
+  switch (format) {
+    case FORMATS.CLAUDE:
+      injectClaudeSystem(body, prompt);
+      return;
+    case FORMATS.GEMINI:
+    case FORMATS.GEMINI_CLI:
+    case FORMATS.VERTEX:
+    case FORMATS.ANTIGRAVITY:
+      // Antigravity wraps Gemini shape in body.request → injectGeminiSystem handles it
+      injectGeminiSystem(body, prompt);
+      return;
+    default:
+      // OpenAI and OpenAI-shaped formats (responses/codex/cursor/kiro/ollama)
+      injectMessagesSystem(body, prompt);
+  }
+}
+
+// OpenAI-shaped: messages[] (chat) or input[] (responses) or instructions (responses string)
+function injectMessagesSystem(body, prompt) {
+  // OpenAI Responses API: top-level string field
+  if (typeof body.instructions === "string") {
+    body.instructions = body.instructions
+      ? `${body.instructions}${SEP}${prompt}`
+      : prompt;
+    return;
+  }
+
+  const arr = Array.isArray(body.messages) ? body.messages
+    : Array.isArray(body.input) ? body.input
+    : null;
+  if (!arr) return;
+
+  const idx = arr.findIndex(m => m && (m.role === "system" || m.role === "developer"));
+  if (idx >= 0) {
+    appendToOpenAIMessage(arr[idx], prompt);
+  } else {
+    arr.unshift({ role: "system", content: prompt });
+  }
+}
+
+function appendToOpenAIMessage(msg, prompt) {
+  if (typeof msg.content === "string") {
+    msg.content = `${msg.content}${SEP}${prompt}`;
+  } else if (Array.isArray(msg.content)) {
+    // Responses-style array of parts {type:"input_text"|"text", text}
+    msg.content.push({ type: "input_text", text: prompt });
+  } else {
+    msg.content = prompt;
+  }
+}
+
+// Claude shape: body.system as string | array of {type:"text", text}
+// Insert before the last cache_control block to keep injection inside the cached prefix.
+function injectClaudeSystem(body, prompt) {
+  if (typeof body.system === "string" && body.system.length > 0) {
+    body.system = `${body.system}${SEP}${prompt}`;
+    return;
+  }
+  if (Array.isArray(body.system)) {
+    const block = { type: "text", text: prompt };
+    let lastCacheIdx = -1;
+    for (let i = body.system.length - 1; i >= 0; i--) {
+      if (body.system[i]?.cache_control) { lastCacheIdx = i; break; }
+    }
+    if (lastCacheIdx >= 0) {
+      body.system.splice(lastCacheIdx, 0, block);
+    } else {
+      body.system.push(block);
+    }
+    return;
+  }
+  body.system = prompt;
+}
+
+// Gemini shape: body.system_instruction | body.systemInstruction | body.request.systemInstruction
+// Each shape: { parts: [{ text }] }
+function injectGeminiSystem(body, prompt) {
+  const target = body.request && typeof body.request === "object" ? body.request : body;
+  const useSnake = Object.prototype.hasOwnProperty.call(target, "system_instruction");
+  const key = useSnake ? "system_instruction" : "systemInstruction";
+  const sys = target[key];
+  if (sys && Array.isArray(sys.parts)) {
+    sys.parts.push({ text: prompt });
+    return;
+  }
+  target[key] = { parts: [{ text: prompt }] };
+}