feat: opt-in chat bubble layout (closes #336) (#403)

* feat(ui): opt-in chat bubble layout

Closes #336.

Adds a settings toggle that right-aligns user messages and left-aligns
assistant replies. Off by default - the current full-width layout is
friendlier to code blocks and tool output, so bubbles are strictly
opt-in per the maintainer note on the issue.

Wiring follows the existing token-usage / cli-sessions pattern:

- api/config.py: new bubble_layout bool in _SETTINGS_DEFAULTS and
  _SETTINGS_BOOL_KEYS, validated + persisted like the rest.
- static/style.css: .bubble-layout gated selectors using :has() to
  tag msg-rows by .msg-role.user / .msg-role.assistant without any JS
  changes to message creation. User rows get align-self: flex-end,
  max-width: 75%, and a row-reverse header; assistant rows flex-start.
  A 700px media query widens the max to 92% on narrow screens.
- static/index.html: new checkbox with i18n keys next to the existing
  token-usage toggle.
- static/panels.js: loads the setting into the checkbox, saves it
  back, and toggles body.bubble-layout immediately on save.
- static/boot.js: applies the class on initial load so refreshed
  tabs honor the persisted setting without a flash.
- static/i18n.js: English label + description.

Test suite errors are environmental (test server fails to start on
port 8788 on main as well).

* i18n(es): add Spanish translations for bubble_layout setting

* fix+test: boot.js bubble-layout reset on failure; add 22 tests for issue #336

* docs: v0.50.24 release — version badge and CHANGELOG

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

static/style.css

@@ -350,6 +350,17 @@
   @media(min-width:1800px){.messages-inner{max-width:1200px;}}
   .msg-row{padding:10px 0;}
   .msg-row+.msg-row{border-top:none;}
+  /* Bubble layout (issue #336): opt-in chat-bubble look with user messages right-aligned
+     and assistant messages left-aligned. Uses :has() to tag rows by role without JS
+     changes. Full-width by default -- enabled via body.bubble-layout from settings. */
+  body.bubble-layout .msg-row:has(.msg-role.user){align-self:flex-end;max-width:75%;}
+  body.bubble-layout .msg-row:has(.msg-role.user) .msg-body{padding-left:0;padding-right:30px;max-width:none;}
+  body.bubble-layout .msg-row:has(.msg-role.user) .msg-role{flex-direction:row-reverse;}
+  body.bubble-layout .msg-row:has(.msg-role.assistant){align-self:flex-start;max-width:75%;}
+  @media(max-width:700px){
+    body.bubble-layout .msg-row:has(.msg-role.user),
+    body.bubble-layout .msg-row:has(.msg-role.assistant){max-width:92%;}
+  }
   .msg-role{font-size:12px;font-weight:500;letter-spacing:.01em;margin-bottom:8px;display:flex;align-items:center;gap:8px;}
   .msg-role.user{color:rgba(124,185,255,0.65);}
   .msg-role.assistant{color:rgba(201,168,76,0.6);}