feat: remove bubble_layout setting end-to-end (#777)

Removes the bubble_layout toggle from Settings, all persistence, CSS, i18n strings, and the UI docs demo. The CSS was already effectively dead. Users with a saved bubble_layout value in settings.json get a clean migration via _SETTINGS_LEGACY_DROP_KEYS.

Credit: @aronprins (PR #760 / #777)

Co-authored-by: aronprins <aronprins@users.noreply.github.com>

static/index.html

@@ -554,13 +554,6 @@
               </label>
               <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_token_usage">Displays input/output token count below each assistant reply. Also toggled with <code>/usage</code>.</div>
             </div>
-            <div class="settings-field">
-              <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
-                <input type="checkbox" id="settingsBubbleLayout" style="width:15px;height:15px;accent-color:var(--accent)">
-                <span data-i18n="settings_label_bubble_layout">Chat bubble layout</span>
-              </label>
-              <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_bubble_layout">Right-align user messages and left-align assistant replies. Off by default to keep code blocks and tool output full-width.</div>
-            </div>
             <div class="settings-field">
               <label for="settingsSidebarDensity" data-i18n="settings_label_sidebar_density">Sidebar density</label>
               <select id="settingsSidebarDensity" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px">