Animate the Cloud Mode viewer session-loading indicator

[warp-agent-staging]

Description

The Cloud Mode / shared-session viewer loading indicator (TerminalView::render_viewer_loading, shown while a shared session viewer or a conversation transcript is loading) was a static refresh.svg icon and "Loading session..." text — nothing animated, making it unclear the session was actively loading.

This replaces the static icon + text with shimmering_warp_loading_text, the shimmering-text loading indicator already used elsewhere in the app for the same purpose (e.g. the remote-server loading footer, the cloud mode agent loading screen). The indicator now animates continuously while loading and disappears once loading completes, reusing the existing ShimmeringTextElement animation/repaint plumbing rather than hand-rolling a new one.

Reported directly in Slack (no tracked issue): https://warpdev.slack.com/archives/C0BDQDW8V5E/p1787693813417159

Linked Issue

• N/A — no tracked GitHub issue; reported directly in Slack.

Testing

This is a pure animation/visual change with no new branching logic, so no automated test was added (exempt as a visual/copy-adjacent change per repo conventions).

Verified with cargo check -p warp --lib (compiles cleanly), cargo clippy -p warp --lib -- -D warnings (clean), and cargo fmt -p warp -- --check (clean).

Visually verified by driving a TerminalView into the exact "loading conversation transcript" state that renders this indicator (via the existing production PaneGroup::add_loading_conversation_pane API, never resolved with real data) inside the repo's crates/integration real-display recording harness, and recording it. The recording confirms the "Loading session..." text shimmers continuously (highlight sweeping across the text) rather than staying static. This verification scaffolding was not committed.

• I have manually tested my changes locally (via the integration test recording harness described above, since there's no local backend to drive a live Cloud Mode session in this environment).

Screenshots / Videos

The recording below shows the shimmer animation sweeping across the "Loading session..." text.

Computer-use video recordings

View video recording - Recording showing the animated (shimmering) "Loading session..." indicator in the Cloud Mode / shared-session viewer loading state, captured via the crates/integration real-display test harness by driving TerminalModel into the loading-conversation-transcript state through the existing production PaneGroup::add_loading_conversation_pane API (no test-only production code added).

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-IMPROVEMENT: The Cloud Mode session-loading indicator now animates to make it clearer that a session is loading.

[warp-agent-staging]

This PR was generated with Warp.

Comment @warp-factory on this PR to send it follow-up work.