fix(core): prevent CSS injection via rule-breaking declaration values - #5319

Closes #5300.

When UnoCSS scans untrusted, user-generated content, a value that contains {/} can close its own rule and open a new one, injecting arbitrary CSS. Two vectors from the report both hit the same serialization point (entriesToCss, ${key}:${value};):

• Arbitrary bracket value: w-[100px;}body{background:red}]
• Poisoned theme value: colors.custom = 'red;}body{background:red}' → text-custom

Both produced:

.text-custom{color:red;}body{background:red};}

Rather than patching one handler, the guard lives in core's entriesToCss, which every rule body funnels through, so it covers arbitrary values, theme values, and custom rules alike. A declaration whose value contains a top-level { or } (outside a quoted string) is dropped.

Only {/} are rejected, not ;: cross-rule injection requires opening a new block, whereas a lone ; only appends declarations to the same selector (no escalation) and appears in legitimate output (e.g. preset-wind4's bg-conic-*). Quoted strings are respected, so content-["a;b{c}"] still works.

The playground/config parts of the report are the sandbox behaving as designed (you author your own HTML/config), so this change targets the library-level injection.

This PR was created with the help of an agent.