Add clipboard paste button to send text dialog

[Marshmellow31]

Summary

Adds a Paste button to the send-text dialog so users can paste clipboard text directly into the message field.

Closes #492

Changes

• Added a Paste button to the send-text dialog
• Added clipboard reading using navigator.clipboard.readText()
• Preserved the current caret/selection when clicking Paste
• Added graceful handling when clipboard access is unavailable or denied
• Reused the existing insertion logic for native paste and the new Paste button
• Added the required English localization strings

Behavior

The Paste button:

• inserts text at the current caret position
• replaces selected text when a selection exists
• appends text when no active caret is available
• preserves multiline text and Unicode
• leaves existing content unchanged when the clipboard is empty

Native Ctrl/Cmd + V behavior continues to work normally.

Verification

Tested:

• normal text sending
• clipboard paste into an empty field
• paste with existing text
• selection replacement
• empty clipboard
• multiline text
• Unicode and emojis
• denied clipboard permission
• unavailable Clipboard API
• existing native paste behavior
• drag-and-drop behavior

Also ran:

node --check public/scripts/ui.js
git diff --check

Both completed successfully.