fix(paste): handle only the first paste event the Ctrl+V trap receives

Ctrl+V in the terminal inserted the clipboard text twice. Right-click →
Paste inserted it once.

`_handleImagePaste()` appends a hidden contenteditable div, focuses it, and
reads the clipboard out of the paste event that lands there. Two separate
routes deliver that event for a single keypress. The function issues
`document.execCommand('paste')` itself, which in Firefox dispatches a
trusted paste event and then returns false, because the trap cancels the
event and the command never completes; Chromium and WebKit refuse that
command and dispatch nothing. The keydown's own default action delivers the
other, because xterm calls the custom key handler before its own `cancel()`,
so returning false never calls preventDefault. Firefox therefore ran the
trap's listener twice and both runs reached `terminal.paste()`. The
context-menu paste involves no keydown at all, which is why that path stayed
correct.

The trap now accepts the first paste event and cancels every later one, so
how many paste events a browser delivers no longer changes what the PTY
sees. Measured on a live install, one Ctrl+V each: Firefox two events and
two writes before this change, Chromium and WebKit one and one, and every
engine one write after it.

The `execCommand('paste')` call stays. Stripping it out also ends the
doubling, and all three engines still deliver one event without it, since
`trap.focus()` has already run when the key's default action resolves. It is
kept because the trap technique arrived in #84 for plain HTTP and for
mobile, and a desktop measurement says nothing about real iOS Safari or
Android Chrome: where a browser aims the default action at the element
focused when the keydown began, the command is the only route into the trap,
and the trap is the only place clipboard image blobs are read.

test/image-paste-trap.test.ts loads image-input.js into a `node:vm` context
with a fake document and fires two paste events at the trap. It covers text
and images, and fails on the old code with the text pasted twice and the
image uploaded twice.

Docs: the invariant goes into docs/architecture-invariants.md as a Terminal
paste section and into CLAUDE.md as a Frontend entry, both recording the
measured event counts and why the redundant call is still there. README.md
and the Keyboard Shortcuts and Input and Voice wiki pages gain a Ctrl+V row,
which all three tables were missing while listing every other clipboard
binding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -302,6 +302,19 @@ Copy goes through `_copyText()` (Clipboard API, then hidden-textarea + `execComm
 
 Feedback is silent on success except ONCE per page load (a feature that works by doing nothing visible cannot otherwise be told from a dead toggle); failures and refusals toast, throttled to 10s so a permanently blocked clipboard cannot paint a toast on every drag. The setting is per-device on both counts required by the settings rule: it is in `displayKeys` AND absent from the `.strict()` `SettingsUpdateSchema` (clipboard access differs by device and by origin, and the plain-HTTP LAN install has no `navigator.clipboard` at all). Tests: `test/terminal-auto-copy.test.ts`.
 
+### Terminal paste (Ctrl+V)
+
+**The Ctrl+V paste trap consumes exactly ONE paste event.** `_handleImagePaste()` (image-input.js) appends a hidden `contenteditable` div, focuses it, and reads the clipboard out of the paste event that lands there. An image becomes an upload whose saved path is typed into the session; text goes through `terminal.paste()`, so the bracketed-paste markers survive. Two independent routes deliver that event for one keypress, and a browser may fire both:
+
+1. **`document.execCommand('paste')`**, which the function calls itself. ⚠️ Its return value proves nothing about whether it fired. Firefox dispatches a trusted `paste` event carrying the real clipboard and still returns `false`, because the trap cancels the event and the command therefore never completes. Chromium refuses the command outright and dispatches nothing.
+2. **The keydown's own default action.** The `Ctrl+V` branch in `attachCustomKeyEventHandler` returns `false`, which stops xterm from evaluating the key into `^V` but does not cancel the DOM event, for the same reason rule 1 of smart copy above spells out. `trap.focus()` has already run by then, so the browser sends its own paste to the trap as well.
+
+Measured against a live install, one `Ctrl+V` each: Firefox delivers two paste events, Chromium and WebKit one. Handling both wrote the clipboard text to the PTY twice, which is why `Ctrl+V` pasted twice while right-click → Paste pasted once. That menu path involves no keydown, so route 2 cannot exist for it.
+
+⚠️ **Route 2 alone is enough in all three engines, so route 1 is redundant where it can be measured.** Strip the `execCommand('paste')` call out and each of the three still delivers exactly one paste event to the trap, Firefox included. The call is kept anyway, because the trap technique was written for the mobile engines that a desktop measurement cannot reach, and a browser that resolves the key's default action against the element focused when the keydown began would send its paste to xterm's textarea instead. Text paste survives that on xterm's own `handlePasteEvent`; image paste does not, since the trap's listener is the only place clipboard image blobs are read. Removing the call is therefore a decision about mobile coverage, not a cleanup.
+
+The guard is a one-shot flag on each trap rather than a browser test or a reading of `execCommand`'s return value, so any count of events produces one insert. Tests: `test/image-paste-trap.test.ts`.
+
 ### Settings surface: App Settings, Session Options, Add Case
 
 **One visual language, three modals.** `#appSettingsModal`, `#sessionOptionsModal` and `#createCaseModal` share the `set-*` surface (left rail, sections of grouped row cards, label + description on the left, control pinned right) through a single `:is(#appSettingsModal, #sessionOptionsModal, #createCaseModal)` scope in `styles.css`. An `:is()` list takes the specificity of its **most specific argument**, and all three arguments are ids, so every rule kept exactly the weight it had when the block was `#appSettingsModal`-only: nothing downstream shifted in the cascade. That property is what let the surface absorb Session Options and then Add Case in two separate commits without a cascade audit each time.