feat(displays): expose DDC/CI contrast, input source, color preset, picture mode

Backend (routes/display.py, services/display_service.py):
- Probe DDC/CI capabilities per monitor at enumeration time
- New endpoints POST /api/display/{contrast,input_source,color_preset,picture_mode}/{id}
- Picture mode goes through raw VCP 0xDC since monitorcontrol has no
  high-level wrapper; labels follow MCCS spec with vendor-friendly fallbacks
- Each capability reports a *_supported flag so the UI can hide rows that
  the hardware does not advertise

Frontend (links.js, app.js, styles.css, locales):
- Monitor cards grow a contrast slider (same editorial copper treatment
  as brightness) and a "PICTURE TUNING" section beneath
- Picture tuning uses the IconSelect widget (matching the audio device
  selector): per-port icons (HDMI, DisplayPort, DVI, VGA, USB-C),
  thermometer for color temps, per-mode icons (movie reel, gamepad,
  ball, etc.) for picture modes
- Humanizers turn SHOUT_CASE enum names into readable labels
  (COLOR_TEMP_6500K -> "6500 K", HDMI1 -> "HDMI 1")
- 14 new i18n keys per locale (en/ru)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -161,6 +161,19 @@
   "display.power_on": "Turn on",
   "display.power_off": "Turn off",
   "display.primary": "Primary",
+  "display.brightness": "Brightness",
+  "display.contrast": "Contrast",
+  "display.tuning": "Picture tuning",
+  "display.input_source": "Input",
+  "display.color_preset": "Color temp",
+  "display.picture_mode": "Picture mode",
+  "display.msg.contrast_failed": "Failed to set contrast",
+  "display.msg.input_changed": "Input source switched",
+  "display.msg.input_failed": "Failed to switch input source",
+  "display.msg.color_changed": "Color preset applied",
+  "display.msg.color_failed": "Failed to apply color preset",
+  "display.msg.mode_changed": "Picture mode applied",
+  "display.msg.mode_failed": "Failed to apply picture mode",
   "browser.title": "Media Browser",
   "browser.home": "Home",
   "browser.manage_folders": "Manage Folders",