fix(mobile): shell Paste inserts a multi-line block as one bracketed paste

[timkjr]

On a phone, the shell accessory bar's Paste overlay sent its text raw and then an Enter (app.sendInput(text) + a delayed \r). Every newline in a multi-line paste reached the shell as Enter, so each line ran as soon as it arrived, followed by one more Enter.

• Paste now sends one write wrapped in ESC[200~ … ESC[201~ with newlines as CR, through the same path Compose already uses (its bracketed write is extracted into _sendBracketedPaste, unchanged for Compose). Paste never presses Enter: the block lands at the prompt for the user to review and run. This applies to a single-line paste too: a pasted one-liner now waits for Enter instead of running.
• In a shell that never enabled bracketed paste (dash, bash before 5.1), tmux strips the markers, so the embedded CRs still act as Enter there: every line but the last runs as before, and the last one waits. No marker bytes reach the shell.
• A paste over the input-frame budget is refused with a toast and keeps its text for trimming (the raw path could exceed the WS frame limit, and its delayed Enter still went out after a refusal).
• The paste, and an image picked from the same overlay, go to the session the overlay was opened for, not whichever tab is active at Send.

The overlay is only reached for shell sessions (the shell bar's Paste button, and Compose redirecting a shell), so agent sessions are unaffected.

Testing: new cases in test/mobile-prompt-composer.test.ts; typecheck, lint, frontend syntax, format check and the full suite green on this branch. Checked by hand on a phone against my own deployment: a multi-line paste lands at the prompt as one block and runs only on Enter. Checked against tmux 3.5a with a client writing the frame into bash, zsh, dash and cat -v: bash and zsh take the block as one edit, dash and cat show no marker bytes.

[timkjr]

One follow-up commit on top, from a second pass over this PR:

• The image button in the same Paste dialog still uploaded to whichever tab was active when the image was picked. It now uses the session the dialog was opened for, like the text Send. There's a new test for it.
• Comments corrected: the size refusal now lives in _sendBracketedPaste, and the "not run" note says it holds only for shells with bracketed paste. In dash or bash before 5.1, tmux strips the markers, so every line but the last still runs, as before.

The description now also says that a one-line paste waits for Enter.

[Ark0N]

Thanks @timkjr, shipped in 1.41.1. One fix at merge: a pasted ESC[201~ can no longer end the bracketed paste early and run the rest of the block without Enter.