Chat backgrounds in the box, undo for the notes assistant, and richer… - #22

… history cards

Four pieces of Appearance and Notes work that grew together.

Chat backgrounds now ship with the app. Settings → Appearance offers a gallery
of three — Golden Gate, Sequoia Trail, Pismo Beach — so a fresh install has
something to pick without going and finding a file first. The gallery is read
from Assets/images/backgrounds at startup rather than listed in code, so a
future release adds one by dropping the file in. Choosing your own image still
works and the two are interchangeable; a tile is a starting point, not a mode.
A fresh install opens on Golden Gate at the usual 30% opacity — first launch
only, so nobody who has already set or cleared a background gets re-skinned by
an update.

The notes assistant can be undone. A gold undo arrow appears in the note header
after the assistant inserts or replaces text. Ctrl+Z cannot do this job:
assigning the editor's text resets the TextBox's own undo history, so the one
edit you did NOT type by hand was the one the control could not reverse, and a
Replace could take a whole note with it. The offer covers the assistant's last
edit only and retires as soon as you type, since restoring the earlier buffer
would otherwise discard whatever you had written on top of it. Inserted text
also opens its own line now instead of gluing onto whatever you were half way
through typing.

History cards quote the agent's last reply. A card showed the opening prompt
and the last thing you typed; the last thing the agent SAID is usually what you
actually remember a conversation by. The reply is flattened out of markdown and
clipped to its first couple of sentences so the card does not grow, and the two
quotes are labelled "you" and "reply" so it is clear which voice is which.
Conversations archived before this backfill on the first History open.

Plus: a camera button in each tab header for the snapshot offer that was two
clicks deep in the "…" menu; the snapshot card reads as a card floating over
the chat rather than blending into it, with the raised shade derived per theme
from that theme's own accent so new themes get one automatically; and W98 chat
prompts are readable again — gold on silver was 3.21:1, now black window text
at 11.5:1, with the "Show more" toggle going from 1.44:1 to 4.77:1.

223 Desktop tests pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>