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>

Dockerfile

@@ -8,7 +8,7 @@ RUN npm install --no-audit --no-fund
 COPY frontend/ ./
 # Version shown in the header: the git tag for releases (set by CI), otherwise "<package version>-dev"
 ARG APP_VERSION=""
-RUN npx tsc --noEmit && npm run test:run && APP_VERSION="${APP_VERSION}" npm run build
+RUN npx tsc --noEmit && npm run lint && npm run test:run && APP_VERSION="${APP_VERSION}" npm run build
 
 # ==============================================================================
 # Stage 2: Go Backend Lint, Unit Tests & Binary Build (Cached Go module & sqlite layer)