UI polish: select popovers, light-theme contrast, hover and tooltip glitches

Several small visual defects noticed while using 0.15.1. Grouped here because they share the same owner (design-system components under src/components/ui/** and their themes).

• Select popovers are misaligned with their trigger. In Settings (e.g. Tool access: Off / Inspect / Edit) the dropdown content opens narrower than the trigger and offset from it, and clips the row it belongs to. Content should match the trigger width and align to it.
• Light theme has low contrast in many places. Secondary text, borders and surfaces in panels, chat and Settings fall below comfortable contrast in light mode. Audit the light token set and fix the worst offenders.
• Variable-name chips use an off-brand purple. Bound-variable pills such as Background var / Border var render in a harsh purple that does not match the rest of the palette.
• Tooltips and hover animations glitch. Tooltips and hover transitions flicker, stick or appear in the wrong place on property controls such as the colour picker.

Screenshots of the select popover, the colour picker and the chat table are in the original report and can be attached here.