feat(fp): WP-10 — CIBG button fidelity

Fix button atom's dead .btn-outline-primary → .btn-secondary; add 'ghost'
variant (.btn-ghost, CIBG-documented). RTE toolbar drops invented
.btn-outline-secondary/.btn-sm for .btn-ghost. file-input already used the
correct vendored .btn-upload pattern from the earlier CIBG UI fidelity
pass — no change needed there (documented as a deviation).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/shared/ui/button/button.stories.ts

@@ -16,4 +16,5 @@ export const Primary: Story = { args: { variant: 'primary' } };
 export const Secondary: Story = { args: { variant: 'secondary' } };
 export const Subtle: Story = { args: { variant: 'subtle' } };
 export const Danger: Story = { args: { variant: 'danger' } };
+export const Ghost: Story = { args: { variant: 'ghost' } };
 export const Disabled: Story = { args: { variant: 'primary', disabled: true } };