Port UI improvements from esp32-web-interface to esp8266-web-interface

[Copilot]

The esp32-web-interface copilot/apply-ui-changes-to-uart-backend branch introduced several UI/UX improvements not reflected in this repo. This PR ports all applicable changes.

data/inverter.js

• Move communication error tracking from getParamList JSON parse failures → sendCmd.onreadystatechange HTTP status — more reliable and fires for every request type
• Remove firmwareVersion < 3.53 fallback in getValues; always use stream command
• Add console.error on param parse failure instead of silent catch
• Improve paramsCache.get() to handle bit-field enums (OR-joined active flag names)

data/ui.js

• Replace async showParamUpdateModal (blocking modal) with sendParameterUpdate — shows a non-blocking green success bar instead
• Dynamic step for numeric inputs: 1 for integers, 0.04 for fractional values (was hardcoded 0.05)
• Pause auto-refresh while a param field has focus; resume on blur
• refreshStatusBox null-guards each field individually rather than returning early on first null
• File upload completion uses showParamSuccessBar instead of modal
• populateSpotValueDropDown moved into updateTables callback; clears options before repopulating to prevent duplicates on refresh
• Navbar collapses to 60px (was 80px); navbarIsBig state tracked explicitly
• Version string: v2.0 → v2.3

data/index.html

• Add #param-success-bar div for non-blocking param update notifications
• CAN modal: rename "Offset" → "Position" for the frame bit-position field; add new "Offset" input row (−128–127) for value offset
• CAN mapping table headers updated to match: Position / Length / Gain / Offset
• Auto-reload toggle moved before spinner in navbar

data/style.css

• Add .param-success-bar styles (green, centered, auto-hidden)

esp8266-web-interface.ino

• Add Cache-Control: max-age=86400 for SPIFFS static file responses
• Remove unused size_t sent variables
• Remove server.client().setNoDelay(1) after server.begin()