fix: stabilize fullscreen volume and expanded swipes

[FoOcean]

Pre-submission checklist

• I have read CONTRIBUTING.md and completed the required checks
• Local and CI checks pass (local checks pass; CI is pending)
• I have completed a self-review
• I confirm that I haven't read the checkboxes and just ticked them all

Change type (select one)

• feat New feature
• fix Bug fix
• docs Documentation or templates
• style Formatting with no behavior change
• refactor Code change that neither fixes a bug nor adds a feature
• perf Performance improvement
• test Test-related change
• chore Build, CI, dependency, or tooling change
• revert Revert
• security Security fix

Scope (select all that apply)

• Application core
• UI style or layout
• Application behavior
• Dependency changes
• Plugin API
• CI/CD configuration
• Documentation
• Other (describe below)

Related issues

Follow-up to #187.

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Description

Summary

Keep the fullscreen volume overlay steady during repeated adjustments, prevent horizontal swipes from collapsing the expanded island, and prepare the bilingual 1.3.9 changelog consumed by the stable release workflow.

Motivation and context

When volume changed during fullscreen auto-hide, each change reapplied an upward velocity and the overlay window could switch between its hidden and normal origins. In the expanded island, pressing the top header or dragging outside the island could collapse it before a horizontal swipe finished.

Changes

• Apply reveal velocity only when a fullscreen volume or brightness overlay first appears, and keep the hidden window origin fixed until the temporary overlay ends.
• Collapse the expanded header only after a stationary tap is released; protect gestures that began inside the island from the outside-click collapse path.
• Add matching English and Chinese v1.3.9 changelog entries, including these fixes, so .github/scripts/extract_changelog.py can produce the official release body.

UI changes

Repeated fullscreen volume adjustments keep the temporary island in one stable position. Quick left/right swipes in expanded mode leave it expanded; a light tap on the header still closes it.

Validation: cargo fmt --all, cargo clippy --workspace --release --offline -- -D warnings, cargo build --release --offline, and the 1.3.9 release-note extractor passed on Windows. Physical fullscreen and touch interaction remain to be checked by reviewers.