Fix browser image paste in remote terminals

[wesm]

Forge now handles image-only browser paste at the web-terminal boundary, so remote Linux terminals no longer ask the host X11 clipboard for browser content.

• Uploads PNG, JPEG, and WebP clipboard payloads to the terminal-owning host, including Fleet HTTP and SSH peers, then pastes readable paths through the existing bracketed-paste flow.
• Keeps text-first paste behavior and supports sequential multi-image paste.
• Preserves macOS terminal bindings by replaying Ctrl+V whenever the clipboard is not image-only or browser clipboard access fails. Alt+V remains terminal-owned.

[roborev-ci]

roborev: Combined Review (8865906)

Image upload handling is generally sound, but three medium-severity issues should be fixed before merging.

Medium

• Consecutive uploads produce an invalid shell token — frontend/src/lib/components/terminal/XtermTerminalPane.svelte:428
  Multiple uploaded paths are sent in consecutive bracketed-paste frames without whitespace, causing shells to concatenate them. Join the quoted paths with spaces in a single paste, or insert separators between frames, and test the combined terminal input.

• Asynchronous clipboard probing can reorder terminal input — frontend/src/lib/components/terminal/XtermTerminalPane.svelte:472
  On macOS, Ctrl+V is replayed only after an asynchronous clipboard read while subsequent keystrokes continue immediately. This can break sequences such as quoted-insert followed by a character. Buffer or serialize terminal input until the clipboard probe completes, then replay Ctrl+V or paste the image path before flushing later input.

• SSH proxy request body is read without a size bound — internal/server/fleetapi/fleet_ssh.go:448
  The raw Adapter().Handle callback uses unbounded io.ReadAll, so MaxBodyBytes does not protect this route and an oversized request can consume arbitrary memory. Read through a MaxImageBytes + 1 limit, return HTTP 413 when exceeded, and add an over-limit SSH fleet-route test.

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Reviewers: 2 done | Synthesis: codex, 10s | Total: 9m21s

[wesm]

I'll probably have to wait for #965 to merge and then rebase this

[mariusvniekerk]

moving to draft to prevent accidental trigger fingers