v1.3.1 — Whole-UI colour theming, three-column Settings, browser settings page

Added

• The analyzer names the board it is actually running on. A P4X (rev-3
  silicon, EV board v1.6) now reads "SpectraLab-P4X" on the status bar, the
  splash and in GET /api/status; the older v1.5.2 board still reads
  "SpectraLab-P4". Detected from the silicon revision at runtime rather than
  from the build environment, so it stays correct down the raw idf.py path,
  which skips tools/check_chip_rev.py.
• Configurable boot splash. Five seconds by default (was a hardcoded 2.5), set
  from the on-device Settings screen or the browser, and Off skips it
  entirely. The credit line pulses rather than sitting still.
• A device settings page in the browser — /settings.html. The REST config
  API has been complete since M4 and no HTML page had ever called it, so every
  setting except the network was device-only. The form is built from a single
  descriptor array rather than 25 hand-written fields.
• Shared navigation across every web page. Navigation used to be hub-and-spoke:
  each page had one link home and none to its siblings, so Files to Network was
  two hops.
• Home button beside Back on the screens reached from Settings, so leaving a
  nested dialog is one tap instead of three.

Changed

• The colour theme now applies to the whole UI. It used to cover only the
  spectrum screen and the status-row buttons; Settings, Wi-Fi setup, the file
  dialogs, the splash and the toast were permanently dark-blue, and their
  dropdowns, buttons, sliders and switches were not styled at all — they
  inherited LVGL's stock theme, which this project builds in its light
  variant. Choosing HIGH CONTRAST turned the spectrum light and left everything
  else dark. The palette moved into a shared ui_theme module which also
  installs an LVGL theme hook, so every widget picks up the scheme as it is
  created — including screens added later.
• Settings is three columns instead of two. The old layout wasted 160 px
  down the middle, gave a 220 px dropdown to text like "50%", and pushed three
  whole groups (SPL Calibration, Settings Profile, Auto Gain) below the 600 px
  fold where they were easy to miss. Everything now fits on one screen. Built
  with flex containers, so a row fills its column by itself.
• Back sits in the top-right button row on every screen that has one, in line
  with the settings gear and the screenshot button, instead of at the bottom of
  a column where it moved whenever a group grew.
• The clock and timezone controls moved off the file browser onto the new
  settings page, where a device-wide setting belongs.
• The theme toggle and the browser-clock push are served once from /app.js
  instead of being pasted into all four pages.

Fixed

• Two long-standing overlaps on the Settings screen: the profile hint wrapped
  into the timezone dropdown, and the Wi-Fi Setup button sat inside the mic
  calibration band despite being unrelated to it.
• The toast overlay was unreadable on the light scheme (fixed dark green on
  dark red); it now takes its colours from the active palette.
• A status-row button created before the spectrum screen came up used hardcoded
  DARK colours until the next theme change.

Note

• settings_t grew a field (splash_seconds), which invalidates the NVS blob.
  Devices without an SD card get a one-time settings reset on first boot; SD
  settings.json keeps every existing value. Expected, not a bug.