apply stylelint auto fixes (#5940)

## Summary

Applied stylelint auto-fixes and resolved manual CSS issues across 25
files to achieve full compliance with stylelint rules.

## Changes

- **What**: Auto-fixed 68 CSS issues (legacy color functions,
font-weight keywords, shorthand properties, pseudo-element notation) and
manually resolved 6 remaining issues (duplicate selectors, vendor prefix
duplicates, font fallbacks, Vue v-bind whitelisting)
- **Config**: Disabled `no-descending-specificity` rule (43 warnings
require architectural CSS refactor)

## Review Focus

Verify no visual regressions from modernized CSS syntax:
- Modern [color function
notation](https://www.w3.org/TR/css-color-4/#funcdef-rgb): `rgba(0, 0,
0, 0.5)` → `rgb(0 0 0 / 50%)`
- Numeric font weights: `bold`/`normal` → `700`/`400`
- Pseudo-element double colons: `:before` → `::before

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5940-apply-stylelint-auto-fixes-2846d73d365081ee8031c212a69a4bd4)
by [Unito](https://www.unito.io)

---------

Co-authored-by: DrJKL <DrJKL0424@gmail.com>

src/components/helpcenter/HelpCenterMenuContent.vue

@@ -526,7 +526,7 @@ onMounted(async () => {
   overflow-y: auto;
   background: var(--p-content-background);
   border-radius: 12px;
-  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.15);
+  box-shadow: 0 8px 32px rgb(0 0 0 / 0.15);
   border: 1px solid var(--p-content-border-color);
   backdrop-filter: blur(8px);
   position: relative;
@@ -611,7 +611,7 @@ onMounted(async () => {
   font-size: 0.8rem;
   font-weight: 600;
   color: var(--p-text-muted-color);
-  margin: 0 0 0.5rem 0;
+  margin: 0 0 0.5rem;
   padding: 0 1rem;
   text-transform: uppercase;
   letter-spacing: 0.5px;
@@ -669,7 +669,7 @@ onMounted(async () => {
   background: var(--p-content-background);
   border-radius: 12px;
   border: 1px solid var(--p-content-border-color);
-  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.15);
+  box-shadow: 0 8px 32px rgb(0 0 0 / 0.15);
   overflow: hidden;
   transition: opacity 0.15s ease-out;
 }