feat(web): redesign desktop UI and interactions

[bobleer]

Summary

• Redesign the BitFun Desktop shell, navigation, welcome surface, settings, workspace tools, and core FlowChat experience.
• Unify component-library interaction states, focus treatment, motion timing, pointer-aware hover behavior, and reduced-motion fallbacks.
• Improve keyboard and screen-reader semantics for tabs, selects, menus, dialogs, tooltips, form validation, model selection, and interactive cards.
• Replace broad or layout-heavy transitions in the touched surfaces with focused transform, opacity, color, border, and shadow feedback.

Type and Areas

Type:

UI/UX refactor

Areas:

Web UI, Desktop shell, component library, FlowChat, settings, workspace/editor/git/terminal surfaces

Motivation / Impact

BitFun Desktop had inconsistent density, motion, interaction feedback, and accessibility semantics across high-frequency workflows. This change establishes a calmer, more precise desktop system while preserving the existing information architecture and product behavior.

Users get faster navigation, clearer hierarchy, more consistent controls, reduced visual noise, improved keyboard access, and motion that respects system accessibility preferences.

Verification

• pnpm run build:web passed on the latest upstream/main.
• pnpm run theme:color-audit:all passed for Desktop, mobile, installer, and CLI contracts.
• 19 focused Vitest files passed, 81 tests total, covering navigation layout, settings, component primitives, FlowChat interactions, and the newly rebased dispatch workflows.
• git diff --check passed.
• Manual browser preview checks covered base controls, forms, Select pointer and keyboard selection, Modal focus/semantics, and dark-theme visual hierarchy.

Reviewer Notes

• The branch is rebased onto b5f4db7c4 and retains the latest detached dispatch workflows.
• Theme governance baselines were not raised or relaxed.
• No new dependencies, product strings, locale resources, or platform APIs were added.
• The production build still reports the repository existing Rollup circular-chunk and large-chunk warnings; this PR does not introduce new build failures.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.