chore(frontend): real ESLint instead of a placeholder; fix hook order (FLAW-19)

eslint.config.js with the recommended JS/TypeScript rules and the React
hooks rules; npm run lint runs ESLint and is part of the image build.
Errors fail the build, the 24 exhaustive-deps findings stay warnings.

It found two real bugs: TaskDetailModal and ManageColumnsModal called
their hooks after 'if (!isOpen) return null', so the hook count changed
whenever the dialog opened or closed. Both are now a thin wrapper plus a
content component. Also removes unused imports and dead code (old task
list share handlers, an unused realtime status, an unused rollup).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/OutlinerTree.tsx

@@ -402,7 +402,7 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
 
         // Explicit bullet or numbered list: - item, * item, • item, 1. item, 1) item
         const bulletMatch = strippedLine.match(/^[-*•]\s+(.*)$/);
-        const numMatch = strippedLine.match(/^\d+[\.\)]\s+(.*)$/);
+        const numMatch = strippedLine.match(/^\d+[.)]\s+(.*)$/);
         if (bulletMatch || numMatch) {
           pushCurrentItem();
           afterHeading = false;