fix: Phase 5 - TMUX features backend selector UX

[eyelock]

Testing

• TMUX badge display on cards
• Backend labeling consistency (radio buttons)
• Running session detection for both Direct and TMUX

Fixed

• Removed unused card.isRunning property (dead code never updated)
• Green running indicator now works using hasActiveSession
• Backend selector disabled when terminal session is active
• Clear warning message when backend cannot be changed
• Works correctly for both Direct and TMUX sessions

UX Improvement

Users can no longer change backend while session is running (would have no effect until restart anyway). Orange warning explains they need to restart to change backends.

All tests passing (558 tests).

[claude]

Code Review

Found 2 localization issues that violate CLAUDE.md requirements:

Issue 1: Hardcoded string in backend selector warning

File: Sources/TermQ/Views/CardEditorView.swift:223

Code:

Text("Backend cannot be changed while session is active. Restart the session to change backends.")

Problem: Hardcoded English string should be localized to support all 40 languages.

Reference: implementation-checks.md requires running localization validation when modifying UI strings.

Fix:

Text(Strings.Editor.backendLockedWarning)

Add to Localizable.strings:

"editor.backend.locked_warning" = "Backend cannot be changed while session is active. Restart the session to change backends.";

Add to Strings.swift:

static var backendLockedWarning: String { localized("editor.backend.locked_warning") }

---

Issue 2: Hardcoded string in Next Action warning

File: Sources/TermQ/Views/CardEditorView.swift:454

Code:

Text("Next Action requires \"Enable LLM Prompt Auto-injection\" in Settings → Tools")

Problem: Hardcoded English string should be localized.

Fix:

Text(Strings.Editor.nextActionRequiresInjection)

Add to Localizable.strings:

"editor.next_action.requires_injection" = "Next Action requires \"Enable LLM Prompt Auto-injection\" in Settings → Tools";

Add to Strings.swift:

static var nextActionRequiresInjection: String { localized("editor.next_action.requires_injection") }

---

Context

This PR correctly localized the MCP install button (Strings.Settings.mcpInstall), demonstrating the proper pattern. These two strings should follow the same approach.

All other changes in the PR look good - no bugs or logic issues found.