Add assistant message copy action and harden related test/storage fallbacks

[shivamhwp]

What Changed

Added a copy action for assistant messages in the chat timeline. The control lives in the assistant footer, copies only the assistant response text, stays visible but disabled while the message is still streaming, and becomes clickable once the response is complete.

The shared message copy button was also extended so it can be rendered as a compact muted button for this footer treatment. Tests were updated to cover the assistant copy visibility and streaming-disabled behavior.

Why

The app already supported copying code blocks and user messages, but not full assistant responses.

This approach keeps the behavior predictable by sourcing the copied content directly from the assistant message text already stored in the timeline, so it does not include work-log entries, command metadata, timestamps, or diff UI. Keeping the button disabled during streaming also avoids partial-copy behavior and makes the interaction state explicit.

UI Changes

This adds a small muted copy button next to the assistant message timestamp in the footer. It is disabled while the assistant is streaming and enabled after the response completes.

Before

After

Fixes #1164

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Add copy action to assistant messages and harden storage/SSR fallbacks

• Adds a MessageCopyButton to assistant message rows in MessagesTimeline.tsx: visible and enabled for completed messages, visible but disabled while streaming, hidden when the response is empty; success/error toasts are shown on copy.
• Introduces resolveAssistantMessageCopyState in MessagesTimeline.logic.ts to centralize copy visibility, disabled state, and text resolution for assistant messages.
• Extends MessageCopyButton with disabled, label, size, variant, onCopy, and onError props.
• Hardens useTheme.ts and terminalStateStore.ts against SSR and non-browser environments by gating window/document/localStorage access and falling back to in-memory storage or stable defaults.

^{Macroscope summarized 928a793.}

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