Each browser keeps its own theme, in a cookie (#69)

The theme was kept on the account, so every browser signed in as the
same person got the same one: no Glass on the phone with Dracula on the
desktop. It is now the ipx_theme cookie (<theme>.<mode>), written by
theme.ts, and read by the server to draw the page in it from the first
frame as before. /api/me no longer reports or takes a theme, and
set_theme is gone.

A browser with no cookie yet is sent the theme the account kept, and
takes it as its cookie on that first load, so nobody loses their choice
in the move. users.theme and theme_mode are only read now, for that.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Each browser keeps its own theme, so a phone and a desktop can differ. A browser that has not
+  chosen one yet starts from the theme your account had.
 - The Modern theme takes its colours from the new logo: its navy, the blue of its bars and the
   orange of its needle.
 - The logo follows the page: the dark version in a dark theme, the light one in a light theme.