fix(tui): never render dead code-block copy buttons

[dgageot]

The ⎘ copy affordance rendered by FastRenderer on every fenced code block was only wired up for hit-testing in assistant message views. In reasoning blocks, elicitation dialogs, leantui, shell output, and welcome messages the icon appeared but clicking it did nothing.

The fix adds a HideCopyIcon option to FastRenderer (exposed as markdown.NewRendererWithoutCopyIcon). When hidden, the icon renders as plain padding and records no CodeBlock hit target. Reasoning blocks, elicitation dialogs, and leantui — which never participate in code-block click handling — now use this renderer. Shell-output and welcome message views gain functional copy icons by populating their codeBlocks slice (accounting for the welcome envelope's line offset) so the existing codeBlockAt click path handles them. A new test, TestCodeBlockCopyIconsAreNeverDead, asserts that every rendered copy icon is clickable and that surfaces incapable of handling clicks render no icon at all.

[docker-agent]

Assessment: 🟢 APPROVE

The implementation is clean and correct. The HideCopyIcon option integrates properly with the parser pool lifecycle, the line-offset calculations for welcome messages (border + padding = 2) and shell output (no envelope) are accurate, and the error paths correctly clear codeBlocks to avoid stale state. The new TestCodeBlockCopyIconsAreNeverDead test provides good coverage of the invariant being enforced.