fix: close the window from the last conversation tab

[wibus-wee]

Problem / pressure

Cmd/Ctrl+W in a Session window opened with modifier-click closed its sole conversation tab and left a draft instead of closing the window. The close resolver accepted but ignored the visible conversation tab count.

Summary

Yield to native window close when one conversation tab remains, including a child or draft, without changing shared Session closure state. Keep focused side-panel close priority, individual closure with multiple conversation tabs, and explicit tab × behavior. Update the bilingual window Spec and decision note, and extend the owning regression suite.

Visual explanation

Cmd/Ctrl+W
  focused side-panel tab → close that tab
  conversation with multiple tabs → close active tab
  conversation with one tab → native window close/hide

Before / after

Before	After
A lone conversation closes into a draft.	Its window closes/hides through the existing native handler; shared tab state stays open.
Primary and auxiliary windows use tab-first handling regardless of count.	Both use the same last-tab window-close rule.

Test plan

• Passed: Oxfmt 0.65.0 check on the three changed TypeScript files; git diff --check.
• Added regression cases for the last parent, child, and draft tab, plus focused and hidden side panels.
• Blocked by missing checkout dependencies: pnpm check (tsgo), pnpm format (oxfmt), and targeted Vitest (vitest).
• pnpm run docs check: 34 pre-existing submodule link errors, no new errors.
• Native Electron interaction has not been tested in this checkout.