feat(workspace): merge bullet on Backspace, render hashtags inline

- Backspace at the start of a non-empty bullet appends its text to the
  previous visible bullet (skipped for blocks with children and for
  code/divider/table/progress blocks).
- Hashtags are rendered inline as clickable chips instead of a badge row
  below the text, so lines no longer grow taller.
- Targeted/highlighted bullets use a neutral background instead of accent.

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

frontend/src/lib/markdown.tsx

@@ -7,11 +7,20 @@ import React from 'react';
  * - ~~strikethrough~~
  * - `code`
  * - [label](url)
+ * - #hashtag (only when `options.onTagClick` is given), rendered inline as a clickable chip
  */
-function renderInlineTokens(text: string): React.ReactNode[] {
+export interface InlineMarkdownOptions {
+  onTagClick?: (tag: string) => void;
+}
+
+function renderInlineTokens(text: string, options?: InlineMarkdownOptions): React.ReactNode[] {
   if (!text) return [];
 
-  const regex = /(`[^`]+`|\*\*[^*]+\*\*|__[^_]+__|~~[^~]+~~|\*[^*]+\*|_[^_]+_|\[([^\]]+)\]\(([^)]+)\))/g;
+  const tagAlt = options?.onTagClick ? '|#[a-zA-Z0-9äöüÄÖÜß_-]+' : '';
+  const regex = new RegExp(
+    '(`[^`]+`|\\*\\*[^*]+\\*\\*|__[^_]+__|~~[^~]+~~|\\*[^*]+\\*|_[^_]+_|\\[([^\\]]+)\\]\\(([^)]+)\\)' + tagAlt + ')',
+    'g'
+  );
 
   const elements: React.ReactNode[] = [];
   let lastIndex = 0;
@@ -24,7 +33,30 @@ function renderInlineTokens(text: string): React.ReactNode[] {
     const token = match[0];
     const key = `${match.index}-${token}`;
 
-    if (token.startsWith('`') && token.endsWith('`')) {
+    if (token.startsWith('#')) {
+      const prevChar = match.index > 0 ? text[match.index - 1] : '';
+      if (prevChar && !/[\s(]/.test(prevChar)) {
+        // e.g. an URL fragment like https://x/#anchor: keep as plain text
+        elements.push(token);
+      } else {
+        const tag = token.slice(1);
+        elements.push(
+          <span
+            key={key}
+            role="button"
+            tabIndex={-1}
+            onClick={(e) => {
+              e.stopPropagation();
+              options?.onTagClick?.(tag);
+            }}
+            className="text-accent bg-accent-dim hover:bg-accent/20 rounded-btn px-0.5 cursor-pointer transition-colors"
+            title={`Nach #${tag} filtern`}
+          >
+            {token}
+          </span>
+        );
+      }
+    } else if (token.startsWith('`') && token.endsWith('`')) {
       elements.push(
         <code key={key} className="px-1.5 py-0.5 rounded bg-panel-elevated text-accent font-mono text-[0.9em] border border-border/50">
           {token.slice(1, -1)}
@@ -92,7 +124,7 @@ function renderInlineTokens(text: string): React.ReactNode[] {
  * - `code`
  * - [label](url)
  */
-export function renderInlineMarkdown(text: string): React.ReactNode[] {
+export function renderInlineMarkdown(text: string, options?: InlineMarkdownOptions): React.ReactNode[] {
   if (!text) return [];
 
   if (text.includes('```')) {
@@ -122,14 +154,14 @@ export function renderInlineMarkdown(text: string): React.ReactNode[] {
           </div>
         );
       } else if (part) {
-        nodes.push(...renderInlineTokens(part));
+        nodes.push(...renderInlineTokens(part, options));
       }
     });
 
     return nodes;
   }
 
-  return renderInlineTokens(text);
+  return renderInlineTokens(text, options);
 }
 
 export function MarkdownViewer({ content, className = '' }: { content: string; className?: string }) {