Rename Character copy to Persona

Issue #234: the "Character" tab and its "Style of response" label made it
unclear that this is where a system prompt is set. Rename the user-facing
labels for clarity:

- "Character" tab + section heading -> "Persona"
- "Style of response" -> "System prompt"
- supporting strings: select placeholder, name placeholder, button/title
  text, toasts, confirm/notice text, the chat-bar indicator tooltip, the
  settings visibility toggle, and the assistant personality picker
  ("Characters" optgroup -> "Personas").

Used "Persona" rather than the issue's suggested "Preset" because the app
already has a distinct, user-facing "Presets" concept (built-in presets
like Code Analyze/Brainstorm/Reason, shown as their own group in the
assistant picker). "Persona" matches what this tab actually creates -- a
named persona with its own memories -- without colliding with that term.

Internal identifiers (element IDs, data-chartab attributes, function names)
and the character_name backend field are intentionally left unchanged so
existing saved presets and JS wiring keep working.

static/js/assistant.js

@@ -180,7 +180,7 @@ function _renderSettingsBody(body, data, tzList) {
       <div class="assistant-field">
         <span style="display:flex;align-items:center;gap:8px;">Personality
           <select id="assistant-character-pick" style="font-size:11px;padding:1px 6px;border:1px solid var(--border);border-radius:3px;background:var(--bg);color:var(--fg);max-width:180px;">
-            <option value="">-- pick from character --</option>
+            <option value="">-- pick from persona --</option>
           </select>
         </span>
         <textarea id="assistant-personality" rows="6" placeholder="Describe the assistant's personality, tone, and behavior...">${_esc(crew.personality || '')}</textarea>
@@ -293,7 +293,7 @@ function _renderSettingsBody(body, data, tzList) {
           allPresets.push(...presetsRaw);
         }
         const allTemplates = Array.isArray(templates) ? templates : [];
-        let opts = '<option value="">-- pick from character --</option>';
+        let opts = '<option value="">-- pick from persona --</option>';
         if (allPresets.length) {
           opts += '<optgroup label="Presets">';
           for (const p of allPresets) {
@@ -304,7 +304,7 @@ function _renderSettingsBody(body, data, tzList) {
           opts += '</optgroup>';
         }
         if (allTemplates.length) {
-          opts += '<optgroup label="Characters">';
+          opts += '<optgroup label="Personas">';
           for (const t of allTemplates) {
             if (!t.system_prompt && !t.personality) continue;
             const name = t.character_name || t.name || 'Unnamed';