fix CRITICAL: ToolCard.tsx hardcoded hex → CSS custom properties

Replaced all 80+ hardcoded hex/rgba values in TOOL_THEMES, ToolCard,
FileEditCard, and TerminalCard with theme-aware CSS custom properties
(--tool-write, --tool-edit, --tool-read, --tool-terminal, --tool-search,
--tool-browser, --tool-git, --tool-thinking, --color-accent-green,
--color-accent-amber, --color-accent-red, --surface-elevated, etc).

The dark theme and warm theme already define these with appropriate per-
theme values. Uses color-mix(in srgb, var(--X) Y%, transparent) instead
of the old rgba(r,g,b,Y%/100) pattern so it works with any CSS variable
value format.

Removed the ToolCardTheme interface (accent/bg/border/iconBg/label) and
getTheme() helper in favor of a cssVar+label lookup.

Author: patil-shubham-dev