feat: add inactive window overlay to prevent misdirected shortcuts

[j4rviscmd]

Summary

Adds a full-screen modal overlay that appears when the Coderm window loses focus, blurring and dimming the workbench behind a centered "Not Active" card. This makes it obvious at a glance that keystrokes are no longer reaching this window, preventing accidental shortcut input into the actually-focused window — a frequent pitfall in keyboard-centric (Neovim/Vim) workflows.

Changes

• New file src/vs/workbench/contrib/coderm/browser/inactiveOverlay.ts: Workbench contribution tracking window focus via IHostService.onDidChangeFocus and toggling a full-screen overlay (blurred + dimmed backdrop with a glass card). Follows the cursorAutoHide.ts template structure.
• coderm.contribution.ts: registers the new contribution via side-effect import (alphabetical).
• README.md / README.ja.md: documents the new settings (kept in sync).

New settings

Setting	Type	Default	Description
coderm.inactiveOverlay.mode	string	"on"	on (blur + dim) / off (disabled) / blur-off (dim only, no blur)
coderm.inactiveOverlay.delay	number	300	Delay (ms) before showing the overlay after focus loss (0–5000)
coderm.inactiveOverlay.label	boolean	true	Show the centered "Not Active" card
coderm.inactiveOverlay.dimming	number	0.45	Backdrop darkness 0–1 (applies to both on/blur-off)

Implementation notes

• Focus is tracked via IHostService.onDidChangeFocus only (covers main + auxiliary windows, already debounced via Event.latch); raw DOM blur/focus is intentionally avoided to prevent focus-recursion flicker.
• The overlay holds no focusable element (tabindex-free), so focus is never stolen — the OS-level focus change is left to happen naturally.
• The show path is delayed (delay ms, default 300) to avoid flicker during brief focus switches (e.g. Cmd+Tab); focus restoration hides immediately and cancels any pending show.

Verification

• Compiles cleanly (npm run compile, 0 errors)
• Manually verified: overlay appears on focus loss, disappears on focus restore
• mode: blur-off shows dim-only; mode: off disables entirely
• label: false hides the card; dimming adjusts darkness in real time

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