fix(frontend): unify quiet raised control styling

[hmans]

Why

• Buttons looked more raised than the composer and bottom controls in Kinda 3D mode

What changed

• Share quiet shell lighting across buttons, Select fields, and selected toolbar controls in all depth modes; remove cast shadows, inset press effects, decorative secondary-button borders, and duplicate overrides
• Add a depth comparison story, correct Select story depth values, and update the design-system guide

API compatibility

• No public API or protocol behaviour changed.

Test plan

• Passed: mise lint-frontend, Svelte analyzer for both changed stories, and git diff --check
• Passed: mise x -- pnpm exec vitest run --project=client src/lib/ui/form/Button.svelte.spec.ts src/lib/ui/form/Select.svelte.spec.ts src/lib/ui/Dialog.svelte.spec.ts — 36 tests
• Passed: mise x -- pnpm exec vitest run --project=storybook src/lib/ui/form/Button.stories.svelte src/lib/ui/form/Select.stories.svelte src/lib/ui/HeaderIconButton.stories.svelte src/lib/ui/Dialog.stories.svelte src/lib/ui/ToggleChip.stories.svelte src/lib/ui/PillButtonGroup.stories.svelte — 39 tests
• Chrome DevTools review: light/dark themes, all depth modes, control states, and the sign-out Storybook dialog at 1280 px and 390 px; full frontend suite and production build not run