ci: add Ruff lint rules, pre-commit hooks, and lint job to CI

[korbonits]

Summary

• Add [tool.ruff.lint] to pyproject.toml — the [tool.ruff] section existed with formatting config only (line length, indent width); this PR adds actual lint rules
• Ignore F722/F821 globally: these are false positives from jaxtyping annotations (Float[Array, "b n h d"] etc.) throughout the flax/ modules
• Add .pre-commit-config.yaml with Ruff lint + format hooks
• Add a lint job to .github/workflows/main.yml using astral-sh/setup-uv (uv is already used in the build job)
• Auto-fix 64 violations (unsorted imports, unused imports, f-string placeholders)

Motivation

The project already uses Ruff for formatting but has no lint rules enforced, no pre-commit hooks, and no lint step in CI. This PR completes that setup with a minimal, non-intrusive ruleset (E/F/I) that won't disrupt the existing Google 2-space indent style.

What's not included

• No type checking — out of scope
• No changes to existing build or manual_publish workflows
• No new rules beyond the initial E/F/I selection

Testing

Ran ruff check . and ruff format --check . locally — all checks pass.

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