Let messages follow the app theme, at the user's choice

Messages render on a white card in every theme. That is deliberate for mail
that styles itself, but #4 points out the case it gets wrong: a message with
no styling of its own has nothing worth preserving, and flashing white at
someone reading in the dark is a real cost.

Appearance gains a switch under the theme cards, off by default so the
current behaviour is unchanged. With it on, HTML mail that declares no
colours follows the app theme; mail that sets a background or text colour
still gets the light card it was designed for, because half-darkening someone
else's design is worse than leaving it alone. Plain-text mail already
followed the theme and is untouched by the switch.

The themed palette is expressed in the app's own custom properties, which
cross the shadow boundary, so switching theme repaints open messages without
re-rendering them, and the accent-coloured link stays consistent. The host
element takes color-scheme: inherit so form controls and scrollbars inside a
message match too.

htmlDeclaresColors covers bgcolor attributes, <font color>, and colour or
background declarations in style attributes and <style> blocks, while
ignoring near-misses like border-color and ?color= in a URL.

Closes #4

README.md

@@ -29,6 +29,7 @@ ihasmail is a JMAP-first web client: mail, calendars, contacts, files, filters a
 - Archive / delete / spam / star / mark read / move / labels (IMAP keywords with colours) with **Undo**
 - **"Filter messages like this…"** from the message context menu: creates a Sieve rule pre-filled from the sender/list (target folders can be created on the fly), and can **apply it immediately to the existing messages in the folder** (evaluated client-side, actions applied via JMAP)
 - Safe HTML rendering: DOMPurify sanitisation inside a Shadow DOM, **remote images blocked by default** with a per-sender allow-list and an optional **privacy image proxy** (like Gmail's)
+- Messages sit on a light card by default, untouched as the sender designed them. *Appearance › Apply the theme to messages too* lets them follow the app's light/dark theme instead — plain-text mail always does, and with the option on so does HTML mail that brings no colours of its own; mail that styles itself is still left alone
 - Attachments: previews for images/PDF/text, download all, inline `cid:` images, `.eml` export, *Show original*, header viewer
 - Invitations: `.ics` parts render as an invite card with **Yes/Maybe/No** RSVP (via `CalendarEvent/parse` + iTIP); `.vcf` parts offer *Add to contacts*; `List-Unsubscribe` one-click
 - Search with Gmail operators (`from:`, `to:`, `subject:`, `has:attachment`, `is:unread`, `is:starred`, `in:`, `label:`, `before:`, `after:`, `larger:`, `smaller:` …) plus an advanced-search panel