feat(guid): polish assistant homepage UX and agent switching (#1941)

* feat(guid): polish assistant homepage interactions and agent switcher

* refactor(guid): replace green dot with agent logo in preset agent tag, add split-button agent switcher

- Replace active dot with agent backend logo (Gemini, Claude, etc.) in PresetAgentTag
- Add inner divider between agent logo and assistant avatar for visual clarity
- Left area of tag triggers agent type switcher dropdown; right × always closes
- Lift agent switcher logic from AssistantSelectionArea to GuidPage
- Add border to PresetAgentTag for better theme/dark mode visibility
- Add :root block to retroma-obsidian-book-2.css to satisfy preset CSS test requirements
- Register retroma-obsidian-book-2 in cssThemePresets test expected list

* fix(tests): add retroma-obsidian-book-2-1-dark to preset CSS test whitelist and add required :root block

* feat(guid): expand/collapse assistant description inline, show details button only when expanded

* feat(guid): reset agent selection and input state on new chat navigation

When user clicks "New Chat" from the sidebar, navigate to /guid reuses the
mounted GuidPage component so state was not cleared. Use location.key in a
useEffect to reset selected agent, input, and description expanded state.

* fix(tests): add retroma-nocturne-parchment to preset CSS test whitelist and add [data-theme=dark] block

* feat(guid): show toast after manually switching preset agent type

* feat(guid): refine homepage hero, sidebar, search modal, and settings UI

- Restructure hero title row with back button, edit button, and agent type
  dropdown using unified BUILTIN_AGENT_OPTIONS and useAssistantBackends
- Add inline description expand/collapse with overflow detection
- Add agent logo icons to all agent type selectors across homepage and settings
- Add Down chevron indicators to model/agent selector buttons
- Implement collapsed sidebar layout with centered icon-only buttons
- Reduce expanded sidebar element sizes for better proportion
- Replace workspace collapse chevron with folder open/close icons
- Compact search modal input (52px → 38px) with rounded corners
- Add hover-based expand/collapse to SkillsMarketBanner
- Move custom model hint from add-model modal to model list header
- Remove agent tab content default padding, unify form item spacing
- Route AgentPillBar + to settings/agent?tab=remote

* style: auto-format theme preset CSS files

* fix(guid): use i18n name for assistant hero title

* fix(i18n): remove duplicate shortTitle keys in conversation locale files

* fix(tests): update AgentPillBar test for remote agent tab navigation

* fix(guid): address review issues from PR #1941

- Add try/catch to handlePresetAgentTypeSwitch with Message.error toast on failure
- Fix currentPresetAgentType to use selectedAssistantRecord (handles builtin- prefix)
- Extract modal tree into const to eliminate ~90-line JSX duplication

Review follow-up for #1941

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Co-authored-by: Waili(瓦砾) <icey.liu1994@gmail.com>
Co-authored-by: zynx <>