feat(settings): a live toggle for OSC 52 clipboard reads

[kingb]

Adds "Programs can read clipboard" to Settings → Terminal: a toggle for the existing osc52_read config switch, and makes the switch live.

Stacked on #21. Without #21's engine-gate fix, no clipboard read ever reaches the listener, so this toggle would do nothing. This PR's base is #21's branch; once #21 merges, GitHub retargets it to main. Review it separately; merge it after #21.

What changes for a user

• A new Terminal row, off by default. The help text says what it enables (nvim or tmux over SSH pasting from your local clipboard) and the risk (any program in the terminal can then read what you copied).
• It takes effect immediately, including in open panes.

Why "live" matters here

Before, each pane copied osc52_read when it was spawned. For most settings, "new panes only" is fine. For a privacy switch it's the wrong failure: turning reads off would leave every open pane still sharing the clipboard. Now the app holds one shared gate (Arc<AtomicBool> in Shared), seeded from the loaded config, handed to every pane, and updated in adjust_setting alongside the other live side effects. Each read checks it at that moment.

Tests

Test	Proves
clipboard_read_toggle_is_off_by_default_and_mutates_only_osc52_read	the row defaults off, flips only osc52_read, and flips back
osc52_read_survives_a_save_and_load	the setting round-trips through config.toml; an older file without the key loads with reads off
osc52_read_gate_is_live_in_a_running_pane	off → on → off in one running pane, with no restart; "on" returns the clipboard (base64), "off" returns empty
the existing listener tests	now panic if anything reads the clipboard while reads are off

Sabotage-checked: making the listener ignore the gate fails the "on" read. Making it always read trips the "must not be read" panic and fails the "off" read.

Not covered by an automated test: the two lines of app wiring (seeding Shared, updating in adjust_setting). Covering them needs a full WindowState. They're small and visible in the diff; worth a look in review.

Validation

cargo fmt --check, clippy -D warnings and cargo test --all --all-features are clean. CHANGELOG [Unreleased] has an Added entry, above #21's Fixed.

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