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/credentials/index.html

@@ -1,12 +1,8 @@
-{% extends "base.html" %}
-{% block title %}Credentials — alembic{% endblock %}
-{% block content %}
-<div class="page-header">
-  <div>
-    <h1>Credentials</h1>
-    <p class="subtitle muted">Write-only: values are encrypted at rest and never sent back to the browser. Leave a field blank to keep it unchanged.</p>
-  </div>
-</div>
+{% extends "settings/_layout.html" %}
+{% block settings_title %}Credentials{% endblock %}
+{% block settings_content %}
+<h2 style="margin-top:0;">Credentials</h2>
+<p class="muted" style="margin-top:-0.75rem;">Write-only: values are encrypted at rest and never sent back to the browser. Leave a field blank to keep it unchanged.</p>
 
 <div class="cred-grid">
   {% for scope, fields in scope_fields.items() %}