feat: add AI chat widget powered by ADK Go 1.0

Visitors can ask questions about the CV via a floating chat panel.
The agent uses Gemini to answer questions about experience, projects,
skills, and education by querying the cached CV JSON data.

- internal/chat/agent.go: LLM agent with query_cv tool that searches
  CV data by section (experience, projects, skills, etc.) with keyword filtering
- internal/chat/handler.go: POST /api/chat endpoint with session management,
  graceful degradation when GOOGLE_API_KEY is not set
- chat-widget.html: HTMX-powered floating chat panel with Hyperscript toggle
- _chat.css: Responsive chat UI with dark theme support
- Wired into existing architecture via dependency injection (CVHandler,
  routes, main.go) — zero breaking changes, all existing tests pass

templates/partials/widgets/chat-widget.html

@@ -0,0 +1,59 @@
+{{define "chat-widget"}}
+{{if .ChatEnabled}}
+<!-- AI Chat Widget -->
+<button
+    id="chat-toggle-btn"
+    class="fixed-btn chat-toggle-btn no-print has-tooltip"
+    aria-label="Ask about this CV"
+    data-tooltip="Ask AI about this CV"
+    _="on click toggle .chat-open on #chat-panel
+       then if #chat-panel matches .chat-open
+         then set #chat-input.focus to true
+         then call #chat-input.focus()
+       end">
+    <iconify-icon icon="mdi:chat-outline" id="chat-icon-open"></iconify-icon>
+    <iconify-icon icon="mdi:close" id="chat-icon-close" style="display:none"></iconify-icon>
+</button>
+
+<div id="chat-panel" class="chat-panel no-print">
+    <div class="chat-header">
+        <iconify-icon icon="mdi:robot-outline"></iconify-icon>
+        <span>{{if eq .Lang "es"}}Pregunta sobre este CV{{else}}Ask about this CV{{end}}</span>
+        <button class="chat-close-btn"
+            _="on click remove .chat-open from #chat-panel">
+            <iconify-icon icon="mdi:close"></iconify-icon>
+        </button>
+    </div>
+
+    <div id="chat-messages" class="chat-messages">
+        <div class="chat-message chat-agent">
+            {{if eq .Lang "es"}}¡Hola! Puedo responder preguntas sobre este currículum. Prueba a preguntar sobre experiencia, proyectos, tecnologías o formación.{{else}}Hi! I can answer questions about this CV. Try asking about experience, projects, technologies, or education.{{end}}
+        </div>
+    </div>
+
+    <form id="chat-form" class="chat-input-area"
+        hx-post="/api/chat"
+        hx-target="#chat-messages"
+        hx-swap="beforeend scroll:#chat-messages:bottom"
+        hx-indicator="#chat-spinner"
+        _="on htmx:afterRequest set #chat-input.value to ''">
+        <input type="hidden" name="session_id" value="">
+        <input type="hidden" name="lang" value="{{.Lang}}">
+        <input
+            type="text"
+            id="chat-input"
+            name="message"
+            class="chat-input"
+            placeholder="{{if eq .Lang "es"}}Escribe una pregunta...{{else}}Type a question...{{end}}"
+            autocomplete="off"
+            required>
+        <button type="submit" class="chat-send-btn" aria-label="Send">
+            <iconify-icon icon="mdi:send"></iconify-icon>
+        </button>
+        <div id="chat-spinner" class="chat-spinner htmx-indicator">
+            <iconify-icon icon="mdi:loading" class="spin"></iconify-icon>
+        </div>
+    </form>
+</div>
+{{end}}
+{{end}}