Unicode-correct username canonicalisation (NFKC, casefold, mixed-script), terminal width, cluster-safe truncation and slugs

# Task: make the text helpers Unicode-correct Rewrite the four functions in `textnorm.py` (Python stdlib only, `unicodedata` is fine) to these rules: **canonical_username(s)** - used to detect duplicate/impersonating accounts. 1. Remove zero-width/format characters U+200B, U+200C, U+200D, U+2060, U+FEFF and U+00AD (soft hyphen); apply NFKC; casefold (Python `str.casefold()`; note it maps a Greek final sigma ς to σ, so `"Δημήτρης"` -> `"δημήτρησ"`); strip surrounding whitespace. 2. The result must be 3-32 characters, contain only letters (any script, category L*), digits (Nd), `.`, `_`, `-`, and must not mix scripts: all letters must come from one script among Latin, Greek, Cyrillic (judge by `unicodedata.name` starting with `LATIN`, `GREEK`, `CYRILLIC`; letters from any other script are allowed only if all letters are from that same other script, judged by the first word of the name). Otherwise raise `ValueError`. Examples: `" Ｊｏｈｎ_Doe "` -> `"john_doe"`; `"STRASSE"` and `"straße"` both -> `"strasse"`; `"pаypal"` (Cyrillic а) -> ValueError. **display_width(s)** - terminal columns: characters with East Asian Width `W` or `F` count 2; combining marks (categories Mn, Me), format chars (Cf) and the zero-width chars above count 0; control characters (Cc) count 0; everything else 1 (so East Asian Width `A`/`N`/`Na`/`H` count 1: `…` is 1, `❤` is 1, each regional-indicator letter is 1 so a flag pair is 2, and U+FE0F is 0 as a combining mark). **truncate(s, width, ellipsis="…")** - if `display_width(s) <= width` return `s` unchanged. Otherwise return the longest prefix such that display_width(prefix) + display_width(ellipsis) <= `width`, never splitting a base character from its following combining marks/ZWJ/variation selectors (U+FE00-FE0F), never splitting a regional-indicator pair (flag) and never leaving a dangling ZWJ (U+200D) at the end of the prefix. Then append ellipsis. If even the ellipsis doesn't fit, return "". **slugify(s)** - NFKD, drop combining marks, map `ß`->`ss`, `æ`->`ae`, `œ`->`oe`, `ø`->`o`, `ł`->`l`, `đ`->`d`, `þ`->`th` (and their uppercase forms), lowercase, then every run of characters other than `a-z0-9` becomes a single `-`; strip leading/trailing `-`. If longer than 60 characters, cut to 60 and then, if the cut fell inside a word (the 60th and 61st characters are both letters/digits), back off to the last `-` within the 60 (when there is one); finally strip trailing `-`. Examples: 20 x "word " -> 12 x "word" joined by `-`; `"abc " + 70 x "y"` -> `"abc"`; 70 x "x" -> 60 x "x". Empty result -> `"n-a"`. No network.