Style pane header overflow menu

[zachbai]

Description

• Add the shared menu drop shadow to pane header overflow menus so they match other floating kebab menus.
• Add representative leading icons to pane header overflow actions, including share/link/desktop/stop sharing and maximize/minimize.

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

• I have manually tested my changes locally with ./script/run
• cargo check --manifest-path /workspace/warp/Cargo.toml -p warp
• cargo fmt -p warp
• cargo clippy --manifest-path /workspace/warp/Cargo.toml -p warp --lib -- -D warnings
• Computer use launched the local app with cargo run -- --api-key $STAGING_USER_WARP_API_KEY, created a split pane, and verified the pane header menu renders leading icons, a separator, dark floating menu styling, hover accent, rounded corners, and a visible drop shadow.
• Note: cargo clippy --manifest-path /workspace/warp/Cargo.toml -p warp --all-targets --all-features --tests -- -D warnings was attempted, but failed before checking this change because generated channel config files were missing from OUT_DIR (stable_config.json, dev_config.json, local_config.json, preview_config.json).

Screenshots / Videos

Computer-use visual verification completed; no artifact screenshot attached.

Agent Mode

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

CHANGELOG-NONE

Conversation: https://staging.warp.dev/conversation/2af1863b-b6a9-4d61-85a9-dccc5304acb0
Run: https://oz.staging.warp.dev/runs/019e50f0-01c7-7111-bc1e-e502b9b9a81e
This PR was generated with Oz.