refactor(nav): collapse fleet/device pages into one sidebar entry with internal tab strip

The sidebar had 10 entries with 5 of them (Devices, Seismographs, Sound
Level Meters, Modems, Pair Devices) all about the physical fleet plus
SFM Events as a debug surface.  Operators kept asking "where do I find
BE11529?" without knowing whether it was a seismograph / SLM / modem.

This collapses those 5+1 into a single "Fleet" sidebar entry that opens
into a unified tab strip across the top of the four device pages.  Each
page keeps its existing custom layout (seismograph-specific
calibration/deployment columns, SLM live-status panel, modem pairing
view, all-devices roster).  The strip just provides the navigation +
the "Pair Devices" button as an action.

Sidebar before (10 items):
  Dashboard · Devices · Seismographs · SFM Events · Sound Level Meters
  Modems · Pair Devices · Projects · Job Planner · Settings

Sidebar after (5 items):
  Dashboard · Fleet · Projects · Job Planner · Settings

Changes:
- templates/partials/fleet_tab_strip.html (new): the shared tab strip.
  Auto-detects the active tab from request.url.path.  4 tabs
  (Seismographs / Sound Level Meters / Modems / All Devices) plus a
  "Pair Devices" button on the right.
- templates/{seismographs,sound_level_meters,modems,roster}.html: added
  {% include 'partials/fleet_tab_strip.html' %} as the first thing
  inside the content block.  No other changes to those templates'
  existing layouts.
- templates/base.html: replaced the 6 device-related sidebar links with
  one "Fleet" link to /seismographs.  The Fleet entry is highlighted
  when the current URL is any of /seismographs, /sound-level-meters,
  /modems, /roster, /pair-devices, /unit/*, or /slm/*.
- templates/settings.html: SFM Events moved out of the main nav into a
  new "SFM Admin" card under Settings → Developer.  Daily event
  browsing already lives on project / location / unit pages (Phases
  1+2+3); the standalone /sfm page is now admin / cross-project debug
  surface only.

URLs unchanged — all bookmarks / deep links still work.  /sfm still
serves the standalone page, it's just no longer in the main nav.
Mobile bottom-nav unaffected.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

templates/sound_level_meters.html

@@ -3,6 +3,7 @@
 {% block title %}Sound Level Meters - Seismo Fleet Manager{% endblock %}
 
 {% block content %}
+{% include "partials/fleet_tab_strip.html" %}
 <div class="mb-8">
     <h1 class="text-3xl font-bold text-gray-900 dark:text-white flex items-center">
         <svg class="w-8 h-8 mr-3 text-seismo-orange" fill="none" stroke="currentColor" viewBox="0 0 24 24">