add pipeline task system, agentic music tasks, copy buttons, and pi-bridge cleanup

- Pipeline mode: new task mode that chains agentic tasks sequentially with
  foreach/subdirectory iteration and skip_if conditions
- Pipeline executor backend (WebSocket at /api/tasks/pipeline/ws) with
  support for both Pi and Claude Code models
- Frontend PipelineRunner component with step progress, streaming output,
  and aggregate cost tracking
- New agentic tasks: prepare-discography, fetch-album-info, build-discography
  (pipeline combining both)
- Seed parser extended to handle pipeline steps in frontmatter config
- CopyButton component added to assistant bubbles, error bubbles, and
  tool input/output sections
- Removed obsolete SearXNG/Apify/browser relay code from pi-bridge

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/tasks/build-discography/TASK.md

@@ -0,0 +1,23 @@
+---
+name: Build Discography
+description: Identify all albums in an artist folder, rename directories, fetch detailed info and cover art for each.
+version: 1
+mode: pipeline
+triggers:
+  - type: directory
+inputs:
+  artist_name:
+    type: string
+    description: Artist / band name
+    autofill: entry_name
+steps:
+  - task: prepare-discography
+    inputs:
+      artist_name: ${artist_name}
+  - task: fetch-album-info
+    foreach: subdirectory
+    skip_if: album-info.md
+    inputs:
+      artist_name: ${artist_name}
+      album_name: ${folder_name}
+---