Fix shortcut hints

[premsathisha]

Summary

This PR updates DiffKit shortcut behavior and the places those shortcuts are surfaced:

• Replaces the incorrect Profile hint from G then P to G then U because G then P already goes to Pull Requests.
• Wires G then U to open the signed-in user's profile.
• Makes the mobile profile dropdown item navigate to the profile page instead of being inert text.
• Adds Profile to the command palette with the same G then U shortcut.
• Adds Shift W as a close-current-tab shortcut and documents it in Settings > Shortcuts.
• Improves active-tab closing so it moves to a neighboring tab when possible, and falls back to Overview only when no tabs remain.
• Fixes the tab context menu so “Close tabs to the right” uses the currently right-clicked tab instead of stale ref state.

Why

The profile icon dropdown incorrectly said Profile used G then P, but G then P was already mapped to Pull Requests in the command registry and shortcut settings. Pressing G then P therefore opened Pull Requests, not Profile.

G then U is the clearer replacement because P is already taken by Pull Requests, while U maps naturally to the user profile. That keeps the navigation mnemonic sensible: G for go, U for user.

For tabs, Shift W mirrors the familiar close-tab idea from browser shortcuts without conflicting with browser-level Cmd W behavior. Closing an active tab now goes to the next available tab, or back to Overview when it was the last open tab.

Validation

• pnpm --filter @diffkit/dashboard check
• pnpm --filter @diffkit/dashboard check-types
• pnpm --filter @diffkit/dashboard build

[stylessh]

Please remove png files from changes

[premsathisha]

@stylessh done