fix(ui): unify Tailwind styles and mobile inputs

[fang2hou]

Touching the filename or extension input on iOS could zoom the page, and the placeholders used a different font stack from the rest of the UI. Touch inputs now use 16px text, mobile text fields share an 8px radius, and Send occupies a full row below 640px. History code snippets also gain 4px of space above their container.

The SPA and shared public components now use Tailwind v4 theme tokens and native utilities for fonts, layout, sizes, and interaction states. Global CSS uses base/component layers, with custom utilities for continuous corners and scrollbars. The suffix picker keeps its coordinated close transition and correctly honors reduced motion while open. DEVELOPMENT.md documents the styling conventions, and the public-page test checks the accessible logo link instead of a removed CSS class.

The viewport zoom limits are included, although iOS Safari may ignore them. Broad style changes can affect browser-specific rendering; Chromium checks covered widths from 320px to 1280px, five locale font stacks, dropdown open/search/select/close, mode switching, and card creation/editing/deletion. iPhone Safari was not directly automated.

Validation:

• mise run test: quality gate, production builds, favicon tests, and all 40 Vitest tests passed.
• Svelte autofixer: no new diagnostics across the changed components.
• git diff --check: passed.