feat: batch .md download, multi-select, thumbnails, all OPPORTUNITIES tools

Videos view: multi-select (checkboxes) + select-all/deselect-all + bulk action
bar (Download .md batch, Download thumbnails, Download audio). Per-video buttons:
.md download (done) or Process to .md (pending), clip.

Channels: per-channel pending count + 'Download pending .md' (batches pending
video_ids). Each channel keeps Videos/.md folder/Remove actions.

Backend (jobs.py + api.py):
- POST /api/scrape/batch {video_ids} + POST /api/scrape/video/{id} (on-demand
  .md for any video incl. pending) as SSE-tracked jobs
- GET /api/videos/{id}/markdown (file download)
- POST /api/tools/thumbnails + GET /api/thumbnails/{id} (local cache, offline)
- GET /api/clip/{id}?from=&to= (transcript segment)
- GET /api/stats (aggregate totals)
- POST /api/tools/audio (mp3 job)

Tools view rebuilt as full Knowledge Base grid surfacing every OPPORTUNITIES.md
feature as electable buttons (md download, re-render, export, search, analysis,
thumbnails, audio, clip, stats, cookies, channels, watch, open folders).

Floating job widget (SSE) for any running job. Thumbnails served via
/api/thumbnails/{id} everywhere (works offline once cached).

Launcher: robust Open-Browser helper (3 fallback methods) auto-opens
http://127.0.0.1:<port> on start + on idempotent re-open. data/ gitignored.

scripts/start-server.ps1

@@ -2,6 +2,24 @@ $ErrorActionPreference = 'Stop'
 $root = Split-Path -Parent $PSScriptRoot
 Set-Location -LiteralPath $root
 
+# Robust browser opener — tries several Windows methods until one works.
+function Open-Browser($url) {
+    $opened = $false
+    # Method 1: Start-Process with the URL (default protocol handler)
+    try { Start-Process -FilePath $url; $opened = $true } catch {}
+    if (-not $opened) {
+        # Method 2: explorer.exe with the URL (opens default browser)
+        try { & explorer.exe $url; $opened = $true } catch {}
+    }
+    if (-not $opened) {
+        # Method 3: cmd `start` builtin
+        try { & cmd.exe /c start "" $url; $opened = $true } catch {}
+    }
+    if ($opened) { Write-Host "  [ok] abriendo navegador -> $url" -ForegroundColor DarkGray }
+    else { Write-Host "  [warn] no se pudo abrir el navegador. Abre manualmente: $url" -ForegroundColor Yellow }
+    return $opened
+}
+
 Write-Host ""
 Write-Host "  [yt-scraper] iniciando servidor local..." -ForegroundColor Cyan
 
@@ -15,7 +33,7 @@ if (Test-Path '.run\server.info') {
         $r = Invoke-WebRequest -Uri "http://localhost:$existingPort/healthz" -UseBasicParsing -TimeoutSec 2
         if ($r.Content -match 'ok') {
             Write-Host "  [ok] servidor ya activo en el puerto $existingPort" -ForegroundColor Green
-            Start-Process "http://localhost:$existingPort"
+            Open-Browser "http://127.0.0.1:$existingPort"
             exit 0
         }
     } catch {}
@@ -74,7 +92,7 @@ if (-not $launched) {
     exit 3
 }
 
-Start-Process "http://localhost:$port"
+Open-Browser "http://127.0.0.1:$port"
 Write-Host ""
 Write-Host "  [ok] servidor activo" -ForegroundColor Green
 Write-Host "    URL:   http://localhost:$port"