Improve labels, save recovery, and split-view animation

[sounak1125]

Label assignment and removal now use explicit Add, Apply to all, and remove controls with selection counts and Undo. Labels retain their colors across file reopening and session recovery. Successful saves remove temporary .bak files while interrupted replacements retain recovery protection.

Split view now waits for its pane to initialize, then opens and closes using display-paced animation frames. A stable canvas surface, synchronized clipping, and titlebar movement eliminate repeated canvas clearing and closing distortion. On the tested Windows display, median layout intervals improved from about 31 ms to 6.3 ms, with canvas dimension writes reduced from 22–26 to 2 per transition.

The audit also fixes file-handle cleanup after failed initialization and installer launch failures, and removes confirmed unused imports.

Validation:

• All 68 automated checks passed, including fault injection, save recovery, animation lifecycle, renderer sizing, and installer retry.
• Enhanced split smoke passed with 121 items, checking intermediate display frames, raster scale, post-close settling, reopening, divider movement, item preservation, and renderer errors.
• The rebuilt Windows app passed both split and label smoke tests; packaged runtime files match source.
• Label context menus, view persistence, and multi-window history smoke tests passed.
• Dependency audits reported zero known vulnerabilities. The full coverage and remaining manual installer/update checks are recorded in docs/RELEASE_AUDIT.md.
• CI now runs the split animation smoke on pushes.