feat: refine workbench appearance

[klioen]

Summary

Refines SpireCode's workbench using the interaction and visual principles accepted in the Orca-inspired P0 review. The app chrome is quieter and more neutral, Chat has a more readable measure, System appearance follows the operating system, and primary icon-only controls now expose accessible tooltips.

Related work

• Issue: None
• Accepted intent/spec/plan: docs/orca-inspired-p0/change.md

Changes

• Reworked light and dark theme tokens toward neutral chrome while reserving green for focus, primary actions, and live status.
• Added semantic popover, hover, selected, and focus-ring tokens and applied them to workbench navigation.
• Reduced Chat content width and tightened its prose and process typography.
• Restored persisted System appearance with live prefers-color-scheme updates while keeping the top-bar shortcut two-state.
• Added a reusable tooltip with pointer, keyboard focus, Escape dismissal, viewport-edge alignment, and accessible hidden-state behavior.
• Added regression coverage, localized copy, SDLC documentation, and an Unreleased changelog entry.

Risk and boundaries

• Regression risk: Medium-low; the change touches shared theme CSS and primary workbench controls, with focused and full-suite coverage.
• Security/privacy impact: None. Renderer/Main authority and IPC boundaries are unchanged.
• Data or configuration migration: Existing light and dark preferences remain valid; missing or prior system preferences now resolve dynamically from the OS.
• Platform/package impact: Renderer-only behavior shared by macOS, Windows, and Linux; no native dependency or packaging configuration changed.
• Rollback approach: Revert commit 35fba45; no external or irreversible state is created.

Verification

pnpm check
# 64 test files passed; 415 tests passed; release tests passed

pnpm build
# Renderer, preload, and Electron Main builds passed

pnpm bundle
# macOS arm64 app and DMG built; signature, app smoke, and DMG smoke passed

• Tests cover new or changed behavior.
• pnpm check passes locally, or I explained the exact blocker below.
• I ran pnpm bundle on each affected native target, or this change does not affect packaging/native behavior.
• UI changes include redacted screenshots or recordings when useful.

Contributor checklist

• I read CONTRIBUTING.md and the repository AGENTS.md.
• The change follows an accepted spec/plan, or it is small enough not to require new SDLC artifacts.
• Renderer/Main authority, relative-path validation, Git argument arrays, and credential boundaries remain intact.
• Public docs and CHANGELOG.md are updated for user-visible changes.
• The diff contains no credentials, private data, unrelated generated files, or accidental dependency updates.
• I reviewed the diff for correctness, security/privacy, and compliance with accepted scope.