On screens not yet redesigned with SureColors (the Chats list, etc.), Material's
primary/secondary *container* roles were unset, so they fell back to defaults
that read as blue on the FAB, unread-badge, and avatar surfaces.
- Pin `primaryContainer`/`secondaryContainer` to a neutral Sure surface
(`surfaceInset` + `textPrimary`), and add a `FloatingActionButtonThemeData` so
FABs use Sure's neutral primary action color (`buttonPrimary`). `primary` /
`secondary` stay `link` / `info`, so existing link/accent callers are unchanged.
- Add a `SureLogo` widget that renders `logomark.svg` with the wordmark's
`currentColor` strokes tinted to the theme's secondary text color, and route
every logomark consumer (nav bar, login) through it — so the mark stays legible
in dark mode (the hardcoded grey was too dim) and no caller renders the strokes
as the flutter_svg default (black). The green brand mark keeps its fill.
Refs #2235.
