fix(antigravity): align switch targets with installed products

[andrew05060414]

Summary

• Fix dashboard Antigravity icons/labels to follow the active runtime target instead of always showing Antigravity IDE
• Route auto-switch through installed products (desktop Antigravity when IDE is absent)
• Add configurable multi-surface account sync:
  • active_only (default): switch only the currently selected Antigravity product tab
  • all_targets: sync all enabled surfaces on manual switch
  • Targets: desktop | ide | cli (empty = auto-detect installed products)
• CLI target writes gemini:antigravity system credentials (shared with desktop Antigravity 2.x on Windows)

Context

Fixes scenarios like #931 where users run Antigravity desktop without Antigravity IDE installed, but Cockpit defaulted to IDE paths/icons. Upstream previously merged then reverted Antigravity CLI switching (#1801); this PR keeps CLI as credential-only sync without reintroducing the reverted macOS-only flow.

Follow-up (not in this PR)

• Native Antigravity relay for agy using existing cockpit-cliproxy Antigravity executor (similar to Codex Local Access)
• Seamless in-app switching remains IDE-extension only; desktop still requires credential-only or restart depending on settings

Test plan

• Windows: only Antigravity desktop installed → dashboard shows desktop icon; auto-switch updates desktop account
• Quick settings → set sync mode to all_targets, enable desktop + CLI → manual switch updates credential without IDE restart
• IDE installed + dual no-restart enabled → IDE surface still uses seamless path when selected