#90 light theme: diff text fails AA; form fields look disabled (border 1.2:1)

What

Two light-theme contrast failures were missed by tests/theme_contrast.rs:

1. Diff text on tinted backgrounds: --diff-add on --diff-add-bg = 4.44:1; --diff-del on --diff-del-bg = 4.11:1 (AA needs 4.5).
2. Form fields look disabled: input/textarea use background:var(--bg-alt) with border:1px solid var(--border); light border vs field = ~1.21:1 (UI boundaries need 3:1). The fill vs page is also ~1.07:1, so the field edge is nearly invisible.

Reproduced

Computed from the parsed CSS variables (dark passes); screenshots show the light inputs as flat grey rectangles with no visible border.

Impact

A11y failure and a poor affordance — users cannot tell the fields are editable on light mode.

Fix

• Darken light --diff-add/--diff-del until each passes 4.5 on its background; add those pairs to the contrast test.
• Give form controls a ≥3:1 boundary (darker border or white field on the page background) in both themes.
• Optionally add a test for control-border contrast.