feat(chat): Add undo button to trim conversation to a prior message

Lets you click Undo under a user message to drop it and everything
after it, refilling the prompt with that message's text/images so it
can be edited and resent.

chat/index.html

@@ -97,6 +97,23 @@
                 margin-left: auto;
                 white-space: pre-wrap;
             }
+            .undo-btn {
+                display: block;
+                margin-left: auto;
+                margin-top: 0.4rem;
+                background: rgba(255, 255, 255, 0.15);
+                color: var(--text-user);
+                border: 1px solid rgba(255, 255, 255, 0.4);
+                border-radius: 0.3rem;
+                padding: 0.15rem 0.5rem;
+                font-size: 0.8em;
+                cursor: pointer;
+                opacity: 0.7;
+                transition: opacity 0.2s;
+            }
+            .undo-btn:hover {
+                opacity: 1;
+            }
             .system-message {
                 background: var(--bg-secondary);
                 margin-right: auto;
@@ -283,6 +300,7 @@
                                         <img :src="img.dataUrl" style="max-width: 100%; border-radius: 0.3rem; display: block; margin-bottom: 0.3rem;" />
                                     </template>
                                     <div class="text" x-text="msg.content"></div>
+                                    <button class="undo-btn" title="Trim conversation back to before this message" @click="trimTo(msg)">Undo</button>
                                 </div>
                             </template>
                             <template x-if="msg.role === 'assistant'">
@@ -355,9 +373,8 @@
                       You are a helpful AI assistant.
                       - Do not preach about implications, do not praise excessively.
                       - Keep a critical mindset, do not assume blindly.
-                      - Answer in valid markdown only, no exceptions.
 
-                      - When I ask something about code, or software development, keep in mind that I am a senior backend developer (but do not mention that explicitly), and do not explain excessively, but and challenge my statements with better alternatives if I don't sound right, especially in coding. Do not bring it up otherwise.
+                      - When I ask something about code, or software development, keep in mind that I am a senior backend developer (but do not mention that explicitly), and do not explain excessively, but and challenge my statements where needed with better alternatives if I don't sound right, especially in coding. Do not bring it up otherwise.
                       - Do not give me code if the topic is not about programming or software development.
                   `
                 .replace(/^\s+/gm, "")
@@ -375,9 +392,9 @@
                 sessionId: null,
 
                 models: [
-                    { name: "gpt-5.4-mini", chat: chatWithOpenAI, key: "OPENAI_API_KEY" },
+                    { name: "gpt-5.4-mini", chat: chatWithOpenAI, key: window.env.OPENAI_API_KEY },
-                    { name: "gemini-3.5-flash-lite", chat: chatWithGemini, key: "GEMINI_API_KEY" },
+                    { name: "gemini-3.5-flash-lite", chat: chatWithGemini, key: window.env.GEMINI_API_KEY },
-                    { name: "claude-haiku-4-5-20251001", chat: chatWithClaude, key: "ANTHROPIC_API_KEY" },
+                    { name: "claude-haiku-4-5-20251001", chat: chatWithClaude, key: window.env.ANTHROPIC_API_KEY },
                 ],
 
                 async init() {
@@ -444,7 +461,7 @@
                         const modelObj = this.models.find((m) => m.name === this.activeModelName);
                         await modelObj.chat({
                             model: this.activeModelName,
-                            apiKey: window.env[modelObj.key],
+                            apiKey: modelObj.key,
                             messages: this.messages,
                             signal: this.abortController.signal,
                             onContent: (chunk) => {
@@ -492,6 +509,17 @@
                     });
                 },
 
+                trimTo(msg) {
+                    if (this.isBusy) return;
+                    const idx = this.messages.indexOf(msg);
+                    if (idx === -1) return;
+                    this.messages = this.messages.slice(0, idx);
+                    updateSessionHistory(this.sessionId, this.messages);
+                    this.promptText = msg.content;
+                    this.pendingImages = msg.images ? JSON.parse(JSON.stringify(msg.images)) : [];
+                    this.$nextTick(() => this.$refs.promptInput.focus());
+                },
+
                 restoreConversation(msgs) {
                     this.messages = JSON.parse(JSON.stringify(msgs));
                     this.userHasScrolledUp = false;
@@ -564,11 +592,10 @@
                         "anthropic-version": "2023-06-01",
                         "anthropic-dangerous-direct-browser-access": "true",
                     },
-                    apiKey: window.env.ANTHROPIC_API_KEY,
                 });
             }
 
-            async function chatWithGemini({ onContent, onError, messages, signal, apiKey = window.env.GEMINI_API_KEY, model, ...rest }) {
+            async function chatWithGemini({ onContent, onError, messages, signal, apiKey, model, ...rest }) {
                 const contents = messages.map((msg, i, arr) => {
                     const role = msg.role === "assistant" ? "model" : "user";
                     let parts = [{ text: msg.content }];