refactor: remove unnecessary important declarations

[rdlabo]

Summary

• remove all remaining important declarations
• add state-aware item selector specificity where a normal declaration needs to beat the existing line rule
• leave action-sheet and icon declarations as ordinary author styles so downstream users can override them

Compatibility

• this PR is intentionally stacked on refactor: prefer CSS custom properties over Shadow Parts #39 so it contains only important cleanup
• the resulting combined source is identical to the previously reviewed and Mac-verified implementation
• line classes with and without inset or none states remain covered
• no demo-specific selectors or fixture assumptions
• manager and independent acceptance reviews completed with no blocking findings

Validation

• npm run build
• npm run lint
• git diff --check
• 10 affected Mac Chrome light/dark screenshots matched the before-change baseline

Linux Playwright snapshots were not modified.

[github-actions]

Playwright test results

 78 passed

Details

 78 tests across 1 suite
 1 minute, 25 seconds
 9e05de8
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-40/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.