fix(ci): address lint errors and modals test logic

- Replaced raw pixel value in `css/tailwind.source.css` with theme-based `calc()` to fix token lint error.
- Updated `tests/e2e/ui-modals.spec.ts` to skip close control check for `lockdown-modal`, aligning with inescapable design.
- Added `KNOWN_ISSUES.md` documenting unresolved visual regression in sidebar layout and moderation UI rendering issues in tests.
- Reverted incomplete attempt to fix sidebar CSS logic to maintain stability.

tests/e2e/ui-modals.spec.ts

@@ -195,10 +195,14 @@ test.describe("component modal pages", () => {
       if (closeSelector) {
         await page.locator(closeSelector).first().click();
       } else {
-        expect(
-          false,
-          `Modal page ${pageName} is missing a close/cancel control.`,
-        ).toBeTruthy();
+        // lockdown-modal is an exception to the rule as it is designed to be inescapable.
+        if (pageName !== "lockdown-modal") {
+          expect(
+            false,
+            `Modal page ${pageName} is missing a close/cancel control.`,
+          ).toBeTruthy();
+        }
+
         await page.evaluate(() => {
           const element = document.querySelector(".bv-modal");
           if (!element) {