fix(frontend): bump input placeholder contrast to meet WCAG AA (#12849)

### Why / What / How

**Why:** Placeholder text across the form-input atoms (`Input`,
`Select`, `DateInput`, `DateTimeInput`, `TimeInput`) used
`text-zinc-400` on white — roughly 3.3:1 contrast. Below the 4.5:1 floor
for normal text per WCAG 1.4.3.

Many AutoGPT placeholders carry format-critical guidance (`"Enter
text..."`, `"HH:MM"`, `"Blocks, Agents, Integrations or Keywords..."`,
`"Select options..."`), so the normal-text contrast requirement applies
— these aren't decorative.

**What:** Bump placeholder color from `text-zinc-400` to
`text-zinc-500`.

**How:** One Tailwind class swap across six occurrences in five atom
files. `text-zinc-500` on white is ~4.6:1 (passes AA for normal text)
and remains clearly distinct from real input values (`text-black`).

### Changes 🏗️

- `Input.tsx`, `Select.tsx`, `DateInput.tsx`, `TimeInput.tsx`,
`DateTimeInput.tsx`
- 6 class changes, 5 files, zero structural changes

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [ ] I have tested my changes according to the test plan:
  - [ ] `pnpm types` passes
- [ ] Visual: placeholders are slightly darker but still visually
distinct from typed values. No other style regressions.
- [ ] Run axe DevTools or Lighthouse on a page with empty form inputs —
placeholder contrast now passes AA
- [ ] Dark mode — the change is light-mode only; dark mode is unaffected
(the atoms don't have a `dark:` placeholder override)

Part of a six-branch accessibility pass. Does not depend on the other
five branches.

Co-authored-by: djpjronline-netizen <276112803+djpjronline-netizen@users.noreply.github.com>
Co-authored-by: Nicholas Tindle <nicholas.tindle@agpt.co>