fix(ui): small alignment and size inconsistencies from the Oct 2026 review #1480

Problem

Small inconsistencies found in one visual pass of dev (dark and light themes, 1028 px window):

1. ERD toolbar wraps at ~1000 px and leaves Export alone on a second row.
2. ERD canvas in the light theme shows a lighter rectangle (the canvas boundary) on the page background.
3. Preferences → MCP Server: Copy … config, Regenerate token and Clear are sm (28 px), Edit theme colors… and the folder buttons are md (32 px).
4. Preferences → Appearance: Interface scale and Animate theme changes rows sit about 5 px off the left edge of the other labels; the slider label is not aligned with its slider.
5. Preferences footer: the ghost Close has no visible affordance and extra space on its right.
6. Connection form: the path field (lg, ~37 px) is taller than Browse… (md, ~34 px) in the same row.
7. Dropdown menu is ~12 px narrower than its trigger (Preferences → Theme mode): the right edges do not line up.
8. Disabled Next / Save / Save Changes are a dim teal with low label contrast.
9. SQL toolbar badge reads Transaction — before the first run on SQLite (then Auto-commit).

Scope

One PR; each item either fixed or noted why not.

Found while driving a Linux debug build of dev (UX review, Oct 2026). Part of epic #1331.