feat(settings): configure tmux notification display duration

[TomCC7]

Summary

• add notification_display_duration_ms to settings with validation and a longer default (3000ms), plus General settings UI controls
• pass notification duration through the status poller and completion notification path so all completion-trigger branches use the same send behavior
• update tmux notification command args to use display-message -d <ms> for both session-target and client-target sends, with coverage in arg-builder tests

Verification

• cargo fmt
• cargo test
• cargo build

[codecov-commenter]

Codecov Report

❌ Patch coverage is 30.30303% with 138 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/tmux/mod.rs	33.02%	73 Missing ⚠️
src/app/polling.rs	7.69%	24 Missing ⚠️
src/ui.rs	0.00%	22 Missing ⚠️
src/app/update.rs	18.75%	13 Missing ⚠️
src/app/core.rs	54.54%	5 Missing ⚠️
src/app/mod.rs	50.00%	0 Missing and 1 partial ⚠️

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