Use version range for PostCSS

[RobinMalfait]

This PR fixes an issue where new PostCSS release could lead to type related issues if newer versions change the types.

Right now @tailwindcss/postcss uses a hardcoded PostCSS version. Let's loosen this up and use a semver range instead.

Fixes: #20288

Test plan

1. All tests still pass

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the only change is relaxing an exact PostCSS pin to a compatible range, with no behaviour change for the current development environment.

The change is a one-line edit in the workspace catalog and its corresponding lock file entry. The resolved version in the lock file remains 8.5.15, so nothing changes for local builds or tests. The looser specifier is only visible to consumers of the published package, where it was the source of the reported type errors.

No files require special attention.

_{Reviews (3): Last reviewed commit: "update changelog" | Re-trigger Greptile}

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: aa02d20b-dc20-48d4-9b8a-bcd94ca3a16b

📥 Commits

Reviewing files that changed from the base of the PR and between c8b081d and 1b0e914.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (2)

• CHANGELOG.md
• pnpm-workspace.yaml

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Walkthrough

The postcss catalog entry in pnpm-workspace.yaml is changed from the exact version 8.5.15 to the compatible range ^8.5.15. A corresponding entry is added to CHANGELOG.md under [Unreleased] → Fixed, noting that @tailwindcss/postcss no longer produces type errors with newer PostCSS patch releases.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: switching PostCSS to a version range.
Description check	✅ Passed	The description matches the change and explains the type-error fix with a semver range.
Linked Issues check	✅ Passed	The PR updates PostCSS from a pinned version to a caret range, matching issue #20288.
Out of Scope Changes check	✅ Passed	The changes are limited to the PostCSS version-range update and a related changelog note.

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