Character limit displayed off by one

**Describe the bug**
When writing a post, the progress bar at the bottom shows a limit of 256 characters. However, a message with exactly 256 characters won't send, while a 255 character message does.

**To Reproduce**
Steps to reproduce the behavior:
1. Write a new post with exactly 256 characters
2. Press the send button; nothing happens
3. Delete one character and press send again; it now works

**Screenshots**

This message was 256 characters, but I had to remove the "b" from "dumb" for it to go through.

**Smartphone (please complete the following information):**
- Device: Samsung Galaxy S24
- Android version: 16
- App version: 0.9.4

Reproduce the issue in the Android app using a post of exactly 256 characters, then trace the character-limit display and send validation. Done means the displayed limit matches the accepted maximum and a 256-character post sends successfully; verify that 255-character posts still work.

Written by the indexing model from the issue text.