refactor(ui): three-column layout with left rail + main-view migration (#899)
Unifies the shell into a three-column layout (rail + sidebar + main) matching the
hermes-desktop reference, and migrates every per-item detail/edit surface into a
shared main-view canvas with consistent headers, empty states, and action buttons.
Changes:
- New desktop-only left rail (48px) with 8 nav tabs (chat/tasks/skills/memory/workspaces/profiles/todos/settings)
- Persistent app titlebar (replaces per-chat topbar), active conversation title shown
- All panel detail/create/edit views migrated to #mainSkills, #mainTasks, #mainSettings, #mainWorkspaces, #mainProfiles, #mainMemory
- Settings moved out of modal into main-view page; ESC closes it
- YAML frontmatter rendered in collapsible <details> block in skill detail
- Toasts repositioned from bottom-center to top-right with theme-aware success/error/warning/info variants
- Composer workspace chip split into two-button group: files-icon toggles file panel, label opens workspace picker
- .settings-menu → .side-menu / .side-menu-item (generalised, shared by memory and settings panels)
- i18n: ~25 new keys across en/ru/es/de/zh/zh-Hant for all new form labels, placeholders, and empty states
- Mobile: hamburger in titlebar, slide-in sidebar; box-shadow removed from sidebar
- New regression test: tests/test_settings_navigation_and_detail_refresh.py (9 tests)
Co-authored-by: Aron Prins <pwf.aron@gmail.com>
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