Add support for multi-byte chars like emojis (💡) in light device toggle buttons

[aschober]

Description:

Improve UTF-8 handling for light device toggle button labels (including emojis like 💡) by truncating safely before rendering.

• Add a UTF-8-aware truncation helper that avoids splitting multibyte characters and supports approximated visual-width limits for wider characters like CJK and emoji.
• When no WebButton string exists, fix non-English truncation by replacing the "fixed 1-byte truncation" with UTF-8-safe truncation to get the first, full codepoint from D_BUTTON_TOGGLE.
• When a WebButton string exists, replace the "fixed 3-byte truncation" with UTF-8-safe truncation based on a max of 4 codepoints and max visual width of 4.
• Cache WebButton text once per render block and reuse it (has_web_button) to avoid repeated lookups.

Related issue (if applicable): fixes #

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.10
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass