Restructure Settings nav, modernize import page, dedup/genres UX polish

Settings is now one top-nav entry with Credentials and Jobs as sidebar
sub-pages (jobs moved from /jobs to /settings/jobs). Manual import page
replaces the bare file input and free-text filename field with a drag-drop
dropzone and a proper file-picker table. Genres page drops the "force"
checkbox for two explicit buttons (Preview / Fix genres now). Dedup's
"Scan now" runs both the file-naming and acoustic passes together, and the
table labels which pass caught each candidate instead of showing the raw
pass name. .btn-ghost gets a visible border so it reads as a real button
next to Delete/Danger actions instead of looking unaligned. Job names
throughout the UI are now human-readable instead of raw job_key strings.

Also includes the fpcalc exit-code fix from earlier (fingerprint index
was discarding valid fingerprints on files with a benign decode warning).

app/templates/settings/_layout.html

@@ -0,0 +1,20 @@
+{% extends "base.html" %}
+{% block title %}{% block settings_title %}Settings{% endblock %} — alembic{% endblock %}
+{% block content %}
+<div class="page-header">
+  <div>
+    <h1>Settings</h1>
+    <p class="subtitle muted">Credentials and background jobs.</p>
+  </div>
+</div>
+
+<div class="settings-layout">
+  <nav class="settings-nav">
+    <a href="/settings/credentials" class="{{ 'active' if request.url.path.startswith('/settings/credentials') else '' }}">Credentials</a>
+    <a href="/settings/jobs" class="{{ 'active' if request.url.path.startswith('/settings/jobs') else '' }}">Jobs</a>
+  </nav>
+  <div class="settings-content">
+    {% block settings_content %}{% endblock %}
+  </div>
+</div>
+{% endblock %}