Fixes Gmail's dark theme leaving bright white panels in place.

Gmail ships its own dark theme, but a few of its containers — the message
card, the Reply/Forward bar and the button row inside it — keep a white
background with dark text, which is glaring in an otherwise dark inbox. This
release repaints those surfaces, and lightens the text inside them, using
colours SAMPLED from the page's own dark theme rather than hard-coded greys,
so the patch matches whatever Gmail theme you have chosen.

DarkMode also now refuses to apply its colour-wheel invert to Gmail at all.
Inverting a page that already has a dark theme turns the user's chosen accent
colour into a different hue (cyan renders as brown) while plain dark text
turns white, so it produced a broken-looking inbox. Gmail is left to its own
theme, with only the leftover light surfaces corrected.

Also fixed: late-rendering regions were missed because the page observer was
attached before the document body existed; and the popup reported a stale
version number.

No new permissions. No remote code. Nothing changes on any site unless you
ask for it.

Stops inverting pages that are already dark. DarkMode now reads the page's own
background brightness and color-scheme, and detects when another extension has
already inverted the page — then leaves it alone instead of turning a dark site
pale.

New "Invert anyway" button. When a page reads as already dark, the main button
says "Page already dark" and an override appears, so you can force it if the
judgment is wrong.

Remembered sites back out automatically. If a site you marked "always dark"
turns out to be natively dark once styled, DarkMode reverses itself after load
rather than leaving it washed out.

Larger, clearer toolbar sun: bigger disc with thicker rays reaching the icon
edge, easier to read at 16px.

No permission changes.