Add copy-to-clipboard button in transcript tab

[ComputelessComputer]

Context

The transcript tab has a SelectionMenu for copying selected text, but it only activates in editing mode (isEditing && sessionMode === "inactive"):

// apps/desktop/src/session/components/note-input/transcript/shared/index.tsx
{editable && (
  <SelectionMenu
    containerRef={containerRef}
    onAction={handleSelectionAction}
  />
)}

There is no way to copy the full transcript in one action when in read mode or during an active session.

Problem

Users who want to quickly copy the whole transcript (e.g. paste into another app) have no affordance for it. This gap was previously reported in #2403 and #1245 (both closed), but a "copy all" button was never explicitly added.

Solution

Add a Copy button to the transcript tab toolbar. It should work regardless of session/edit state and copy the transcript as plain text formatted as [Speaker]: [text] per segment. A brief confirmation (icon swap or toast) on click.

This is a stop-gap independent of the transcript editor (#2547). Once that lands, this can be folded into or replaced by the editor's toolbar.