fix: cursor visibility, Left/Right button nav, expand banned password chars

- Fix cursor not appearing in Certificate Details text fields (hint addstr
  was clobbering the cursor position set by place_cursor before refresh)
- Add Left/Right arrow key switching between Continue and Cancel buttons
- Expand password generation banned characters from oO01lI to oO01lIQ5S2Z8B
  (adds visually ambiguous pairs: Q/0, 5/S, 2/Z, 8/B)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_password.py

@@ -1,6 +1,6 @@
 from openvpncertupdate import generate_password
 
-BANNED_ALL       = set("oO01lI")
+BANNED_ALL       = set("oO01lIQ5S2Z8B")
 BANNED_POS2_LAST = BANNED_ALL | {"j"}