Pulls the new demo-shell__toolbar into the chat library visual system:
14px sm font, 8px radii, chat-select primitive replacing the four native
selects, surface-alt secondary fill on the "New conversation" button.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Resolved conflicts in demo-shell.component.{css,html} by taking the
polish-pass values — main only reformatted the same rules with line
breaks inside color-mix calls; polish-pass replaces those rules
entirely with the chat-token recipes.

… swap

The native <select> → chat-select swap broke three e2e files (helpers
using .selectOption() and .toHaveValue() against native selects, plus
.locator('option') for native option children). Updated helpers to drive
the chat-select trigger + .chat-select__option menu, and switched the
toolbar's chat-select menu to open downward (top: 100%+8px) since the
toolbar sits at the page top and the primitive's default upward popover
renders offscreen there. chat-input model picker keeps its upward
default.