feat: right-click context menu in the editor

[fredclausen]

What

Adds a right-click context menu to the editor surface (the second of two context-menu PRs; the sidebar menus landed in #7).

egui's TextEdit ships no context menu of its own, so this adds one rather than replacing anything. Entries:

• Cut, Copy (disabled when there is no selection), Paste, Select all
• Find\u2026, Find and replace\u2026
• Save

How

egui can write the clipboard but exposes no imperative read, which a menu-driven Paste needs. So the clipboard ops are imperative:

• clipboard module \u2014 wraps arboard (already in the tree via eframe; now a pinned direct dep, default features off) for read and write, non-panicking and logging on failure.
• edit_ops module \u2014 pure, char-indexed buffer splices (selected_text, delete_range, replace_range) that back cut/paste/select. Fully unit-tested, no egui or clipboard coupling.
• After an edit the resulting caret is stored back into the live TextEdit state so the cursor follows the change; the editor is refocused.
• A clipboard edit that mutates the buffer is folded into the existing changed-this-frame path, so it persists to the swap file and refreshes search matches exactly like a typed edit.
• Find/replace and Save reuse the existing deferred actions.

Menu choices flow through a deferred EditorAction applied after the editor borrow ends, matching the established MenuAction / TreeAction pattern.

Verification

cargo fmt --check, cargo clippy --all-targets --all-features -D warnings, cargo test --all (113 pass, +12 new across edit_ops and app), and cargo machete all green in the Nix dev shell. Release build compiles.

The only Cargo.lock change is adding arboard to frext's dependency list \u2014 no new packages were resolved, since it was already present transitively.