Let users choose when large pastes become text attachments (1k / 5k / 32 KiB presets)

[danddanddand]

Problem

Pasting a long block of text (logs, stack traces, a file) into the composer only folds it into a pasted-text.txt attachment once it reaches 32 KiB, the threshold from #11442. Most everyday pastes are well under that, so they still fill the whole composer and push the draft out of view.

Other agent apps fold much earlier:

• The Codex CLI collapses pastes over 1,000 characters (LARGE_PASTE_CHAR_THRESHOLD in codex-rs/tui/src/bottom_pane/chat_composer.rs). It expands them back into the prompt on submit.
• The Codex desktop app turns pastes of 5,000 characters or more into a Pasted text.txt file attachment. This is the closest match to how T3 Code folds.

Proposal

Keep 32 KiB as the default, and add a small preset setting for when pasted text becomes an attachment:

• 1,000 characters
• 5,000 characters
• 32,768 characters (default)

On web and desktop it's a row under Settings → General. On mobile it's a Settings → Large pastes screen, because mobile keeps its own device preferences. The existing Cmd/Ctrl+Shift+V paste-as-text bypass is unchanged.

Presets rather than a free number keep validation and labels simple. The default stays the same, so nobody's behavior changes unless they opt in.

I have an implementation ready and will open it as a draft PR linked here.

[danddanddand]

Draft implementation: #21490