fix(tui-v2): add missing keysym.py — tuiapp_v2 ImportError on fresh clone

`tuiapp_v2.py:27` imports `fmt_key`, `fmt_keys` from `keysym`, but the
module file was never tracked. PR #513 (tui v2/v3 fixes) is unusable on a
fresh checkout without this. Adds the cross-platform shortcut formatter
that turns binding strings into mac/Win-Linux key labels (e.g.
`"ctrl+b"` → `"⌃B"` / `"Ctrl+B"`), honoring `GA_KEYSYM_STYLE`.

frontends/keysym.py

@@ -0,0 +1,72 @@
+"""Cross-platform keyboard-shortcut display formatter.
+
+One job: turn a Textual binding string like ``"ctrl+b"`` into a human-facing
+label such as ``"Ctrl+B"`` on Win/Linux or ``"⌃B"`` on macOS.
+
+Binding strings (the *physical* keys Textual captures) are NOT touched —
+this module only formats labels for tips / footers / help panels.
+
+Override with env ``GA_KEYSYM_STYLE=auto|mac|ascii`` (default ``auto``).
+"""
+from __future__ import annotations
+
+import os
+import sys
+
+_STYLE = os.environ.get("GA_KEYSYM_STYLE", "auto").lower()
+IS_MAC = _STYLE == "mac" or (_STYLE != "ascii" and sys.platform == "darwin")
+
+# Modifier display per style. mac uses Apple HIG glyphs; others use words.
+_MOD = {
+    "ctrl":  "⌃" if IS_MAC else "Ctrl",
+    "shift": "⇧" if IS_MAC else "Shift",
+    "alt":   "⌥" if IS_MAC else "Alt",
+    "meta":  "⌘" if IS_MAC else "Alt",
+    "super": "⌘" if IS_MAC else "Win",
+    "cmd":   "⌘" if IS_MAC else "Win",
+}
+
+# Bare-key display. Arrows / slash are universal; rest mac-glyphs vs words.
+_KEY = {
+    "enter":     "⏎" if IS_MAC else "Enter",
+    "tab":       "⇥" if IS_MAC else "Tab",
+    "escape":    "⎋" if IS_MAC else "Esc",
+    "esc":       "⎋" if IS_MAC else "Esc",
+    "backspace": "⌫" if IS_MAC else "Backspace",
+    "delete":    "⌦" if IS_MAC else "Del",
+    "space":     "␣" if IS_MAC else "Space",
+    "up": "↑", "down": "↓", "left": "←", "right": "→",
+    "slash": "/", "underscore": "_",
+}
+
+# Joiner between modifier and key. mac concatenates (⌃B); others use '+'.
+_JOIN = "" if IS_MAC else "+"
+
+
+def fmt_key(combo: str) -> str:
+    """``"ctrl+b"`` → ``"⌃B"`` (mac) / ``"Ctrl+B"`` (Win/Linux).
+
+    Unknown single-char keys are upper-cased (``"b"`` → ``"B"``);
+    multi-char names fall back to the original token unchanged.
+    """
+    parts = [p.strip() for p in combo.lower().split("+") if p.strip()]
+    if not parts:
+        return combo
+    mods, key = parts[:-1], parts[-1]
+    key_disp = _KEY.get(key) or (key.upper() if len(key) == 1 else key)
+    mod_disp = [_MOD.get(m, m) for m in mods]
+    if not mod_disp:
+        return key_disp
+    return _JOIN.join(mod_disp) + _JOIN + key_disp
+
+
+def fmt_keys(*combos: str, sep: str = " / ") -> str:
+    """Join multiple combos: ``fmt_keys("ctrl+j", "ctrl+enter")`` →
+    ``"Ctrl+J / Ctrl+Enter"`` or ``"⌃J / ⌃⏎"``."""
+    return sep.join(fmt_key(c) for c in combos)
+
+
+# Convenience constants for f-string templates.
+CTRL  = _MOD["ctrl"]
+SHIFT = _MOD["shift"]
+ALT   = _MOD["alt"]